Remove QStringView's ushort support [1/3]: add opt-out

The use of ushort as the scalar type to mean QChar dates back to C++98 when char16_t and char32_t did not exist, yet. Now that they do, use of the C++11 character types yields self-explanatory APIs and less implementation work (casts).

The QStringView support for ushort characters was helpful in the run up to QTBUG-110403, but now is the time to retire it.

Acceptance criteria:

• Add a macro, say QT_NO_USHORT_STRINGS that disables QStringView's (and, by extension, QAnyStringView's and, where applicable, QString's) support for ushort as a string
• Make sure QtBase compiles with it
• Make sure all Qt modules compile with it
• Make it the default in all modules
  • exception: when it makes QString BiC, we need to find a way around that (similar to removed_api.cpp)