Remove QStringView's ushort support [2/3]: deprecate it

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 deprecation markers such that, in a QT_NO_USHORT_* build, the use of QStringView (and QAnyStringView, QString, ...) ctors and functions that take or return ushort/ushort* to mean UTF-16 is met with a deprecation warning
• Ditto uint/uint*/char32_t/char32_t*.