Replace uses of QString:::utf16() with QString::(const)data()

The utf16() function guarantees a NUL-terminated UTF-16 string, and will reallocate when it can't be sure (most prominently, from objects constructed fromRawData()). Some code uses this function just because it returns a ushort* instead of a (const) QChar*, to avoid a reinterpret_cast, but invokes the detach() path for common QString objects such as {{QStringLiteral}}s.

So, search for, and replace with QString::(const)data(), all uses of QString::utf16() that don't actually need the NUL-termination.

QStringView::utf16() is not affected, and should stay.

The casual use of utf16() has caused QTBUG-98653. Who knows what else is brewing.