Make QStringBuilder mandatory

QStringBuilder has seen quite a few years of service and its edges should be well-rounded by now. I think it's time to think about ditching the classical operator+ for string-ish-es and only maintain the QStringBuilder-based operators.

The advantages

• QStringBuilder needs only O(N) functions in the number of supported types, classical op+ needs O(N²), even with C++20 magic
• only ¼ of testing needed d/t only one combination as opposed to four
• already now, op% can concatenate many more types than op+
• better performance than nested op+'s

The disadvantages:

• SiC for certain types of assignment
• auto problem (Clazy already warns, IIRC), which can be solved by implementing QTBUG-99291

So while this may not be possible within Qt 6, we should definitely keep it in mind for Qt 7.