chore(generator): Replace deprecated use of "endl" with "Qt::endl"

[jcfr]

This changes the usage of the deprecated global endl with Qt::endl for compatibility with Qt 5.15.2 and above. The approach ensures that Qt::endl is used consistently across all versions of Qt by defining a namespaced alias for Qt versions below 5.14.0.

This follows up on the approach introduced in d11fc4f ("Qt6: QTextStream compatibility (PR-120)", 2023-10-05) by explicitly using Qt::endl everywhere and by flipping the logic to have Qt::endl defined for Qt < 5.14.0

Warnings addressed:

/path/to/PythonQt/generator/typesystem.cpp: In function ‘QString formattedCodeHelper(QTextStream&, Indentor&, QStringList&)’:
/path/to/PythonQt/generator/typesystem.cpp:1900:38: warning: ‘QTextStream& QTextStreamFunctions::endl(QTextStream&)’ is deprecated: Use Qt::endl [-Wdeprecated-declarations]
 1900 |             s << indentor << line << endl;
      |                                      ^~~~
In file included from /path/to/Qt/5.15.2/gcc_64/include/QtCore/qdebug.h:49,
                 from /path/to/Qt/5.15.2/gcc_64/include/QtCore/QDebug:1,
                 from /path/to/PythonQt/generator/typesystem.h:49,
                 from /path/to/PythonQt/generator/typesystem.cpp:42:
/path/to/Qt/5.15.2/gcc_64/include/QtCore/qtextstream.h:293:75: note: declared here
  293 | Q_CORE_EXPORT QT_DEPRECATED_VERSION_X(5, 15, "Use Qt::endl") QTextStream &endl(QTextStream &s);
      |                                                                           ^~~~

[jamesobutler]

@mrbean-bremen Seems like a possible hiccup for the rockylinux CI build. May try retriggering the run to get it passing the check.

[mrbean-bremen]

Yes, I have seen this. This same error happens from time to time, seems to be a problem with the archive.