Doc - Update required clang version for QDoc

With qttools.git at ff3e8e81b284cb9d20bd4d9a31cfe91aad7d646d, compilation of QDoc with LLVM 14 (system) fails:

/src/qt/build/qt$ ninja qdoc
[230/233] Building CXX object qttools/src/qdoc/qdoc/CMakeFiles/qdoc.dir/src/qdoc/clangcodeparser.cpp.o
FAILED: qttools/src/qdoc/qdoc/CMakeFiles/qdoc.dir/src/qdoc/clangcodeparser.cpp.o 
/usr/bin/c++ -DCLANG_RESOURCE_DIR=\"/usr/lib/llvm-14/lib/clang/14.0.6/include\" -DLIBCLANG_VERSION_MAJOR=14 -DQDOC2_COMPAT -DQT_CORE_LIB -DQT_DISABLE_DEPRECATED_UP_TO=0x050000 -DQT_EXPLICIT_QFILE_CONSTRUCTION_FROM_PATH -DQT_NETWORK_LIB -DQT_NO_AS_CONST -DQT_NO_AS_CONST=1 -DQT_NO_CONTEXTLESS_CONNECT=1 -DQT_NO_EXCEPTIONS -DQT_NO_FOREACH -DQT_NO_FOREACH=1 -DQT_NO_JAVA_STYLE_ITERATORS -DQT_NO_NARROWING_CONVERSIONS_IN_CONNECT -DQT_NO_QEXCHANGE -DQT_QMLBUILTINS_LIB -DQT_QMLINTEGRATION_LIB -DQT_QML_LIB -DQT_STRICT_QLIST_ITERATORS -DQT_USE_QSTRINGBUILDER -DQT_WARN_DEPRECATED_UP_TO=0x070000 -D_LARGEFILE64_SOURCE -D_LARGEFILE_SOURCE -I/home/paul/src/qt/build/qt/qttools/src/qdoc/qdoc/qdoc_autogen/include -I/home/paul/src/qt/qt5/qttools/src/qdoc/qdoc -I/home/paul/src/qt/build/qt/qttools/src/qdoc/qdoc -I/home/paul/src/qt/qt5/qttools/src/qdoc/qdoc/src -I/home/paul/src/qt/build/qt/qtdeclarative/src/qml -I/home/paul/src/qt/build/qt/qtbase/include/QtQml/6.7.0 -I/home/paul/src/qt/build/qt/qtbase/include/QtQml/6.7.0/QtQml -I/home/paul/src/qt/build/qt/qtbase/src/corelib -I/home/paul/src/qt/build/qt/qtbase/include/QtCore/6.7.0 -I/home/paul/src/qt/build/qt/qtbase/include/QtCore/6.7.0/QtCore -I/home/paul/src/qt/build/qt/qtbase/include -I/home/paul/src/qt/build/qt/qtbase/include/QtCore -I/home/paul/src/qt/build/qt/qtbase/mkspecs/linux-g++ -I/home/paul/src/qt/build/qt/qtbase/include/QtQmlBuiltins/6.7.0 -I/home/paul/src/qt/build/qt/qtbase/include/QtQmlBuiltins/6.7.0/QtQmlBuiltins -I/home/paul/src/qt/build/qt/qtbase/include/QtQmlBuiltins -I/home/paul/src/qt/build/qt/qtbase/include/QtQml -I/home/paul/src/qt/build/qt/qtbase/include/QtQmlIntegration -I/home/paul/src/qt/qt5/qtdeclarative/src/qmlintegration -I/home/paul/src/qt/build/qt/qtdeclarative/src/qmlintegration -I/home/paul/src/qt/build/qt/qtbase/include/QtNetwork -isystem /usr/lib/llvm-14/include -g -std=c++17 -fvisibility=hidden -fvisibility-inlines-hidden -Wall -Wextra -fno-exceptions -fPIC -Werror -Wno-error=cpp -Wno-error=deprecated-declarations -Wno-error=strict-overflow -Wno-error=implicit-fallthrough -Wno-error=deprecated-copy -Wno-error=redundant-move -Wno-error=init-list-lifetime -Wno-error=format-overflow -Wno-error=stringop-overflow -Wno-error=deprecated-enum-enum-conversion -Wno-error=deprecated-enum-float-conversion -Wsuggest-override -MD -MT qttools/src/qdoc/qdoc/CMakeFiles/qdoc.dir/src/qdoc/clangcodeparser.cpp.o -MF qttools/src/qdoc/qdoc/CMakeFiles/qdoc.dir/src/qdoc/clangcodeparser.cpp.o.d -o qttools/src/qdoc/qdoc/CMakeFiles/qdoc.dir/src/qdoc/clangcodeparser.cpp.o -c /home/paul/src/qt/qt5/qttools/src/qdoc/qdoc/src/qdoc/clangcodeparser.cpp
In file included from /home/paul/src/qt/qt5/qttools/src/qdoc/qdoc/src/qdoc/clangcodeparser.cpp:40:
/home/paul/src/qt/qt5/qttools/src/qdoc/qdoc/src/qdoc/clang/AST/QualTypeNames.h: In function ‘bool clang::TypeName::getFullyQualifiedTemplateName(const clang::ASTContext&, clang::TemplateName&, bool)’:
/home/paul/src/qt/qt5/qttools/src/qdoc/qdoc/src/qdoc/clang/AST/QualTypeNames.h:103:38: error: ‘class clang::TemplateName’ has no member named ‘getAsUsingShadowDecl’
  103 |     if (UsingShadowDecl *USD = TName.getAsUsingShadowDecl())
      |                                      ^~~~~~~~~~~~~~~~~~~~
/home/paul/src/qt/qt5/qttools/src/qdoc/qdoc/src/qdoc/clang/AST/QualTypeNames.h:104:38: error: ‘clang::TemplateName::TemplateName(void*)’ is private within this context
  104 |       UnderlyingTN = TemplateName(USD);
      |                                      ^
In file included from /usr/lib/llvm-14/include/clang/AST/Type.h:22,
                 from /usr/lib/llvm-14/include/clang/AST/DeclarationName.h:16,
                 from /usr/lib/llvm-14/include/clang/AST/DeclBase.h:18,
                 from /usr/lib/llvm-14/include/clang/AST/Decl.h:19,
                 from /home/paul/src/qt/qt5/qttools/src/qdoc/qdoc/src/qdoc/clangcodeparser.cpp:29:
/usr/lib/llvm-14/include/clang/AST/TemplateName.h:197:12: note: declared private here
  197 |   explicit TemplateName(void *Ptr);
      |            ^~~~~~~~~~~~
/home/paul/src/qt/qt5/qttools/src/qdoc/qdoc/src/qdoc/clang/AST/QualTypeNames.h:107:65: error: cannot convert ‘clang::TemplateName’ to ‘clang::TemplateDecl*’
  107 |                                      /TemplateKeyword=/false, UnderlyingTN);
      |                                                                 ^~~~~~~~~~~~
      |                                                                 |
      |                                                                 clang::TemplateName
In file included from /usr/lib/llvm-14/include/clang/Frontend/ASTUnit.h:17,
                 from /home/paul/src/qt/qt5/qttools/src/qdoc/qdoc/src/qdoc/clangcodeparser.cpp:36:
/usr/lib/llvm-14/include/clang/AST/ASTContext.h:2168:55: note:   initializing argument 3 of ‘clang::TemplateName clang::ASTContext::getQualifiedTemplateName(clang::NestedNameSpecifier*, bool, clang::TemplateDecl*) const’
 2168 |                                         TemplateDecl *Template) const;
      |                                         ~~~~~~~~~~~~~_^~~~~~~
[231/233] Linking CXX shared library qtbase/lib/libQt6Qml.so.6.7.0
ninja: build stopped: subcommand failed.