Clarify that "CONFIG += c++latest" is not compiler-specific

According to https://doc.qt.io/qt-6/qmake-variable-reference.html, c++latest means that
"Support for the latest C++ language standard is enabled that is supported by the compiler."

This wording implies that qmake would choose a different standard depending on which toolchain is used. However, this is not the case: It currently unconditionally applies -std=gnu++2b to all GCC and Clang toolchains. This causes errors when using Android NDK 21.3, for example.

error: invalid value 'gnu++2b' in '-std=gnu++2b'

Suggestions

• Remove the phrase "that is supported by the compiler."
• Add a disclaimer that this option might fail on older compilers.