PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
• File the PR against the correct branch: master
• Copy the technical committee to review the pull request if your PR is targeting a particular programming language.

I created an Announcement #8232 regarding Qt6. This PR will remove all deprecated functions in the Qt5 generator.

These are the changes i did to support Qt6:

• qsrand() is now a QRandomGenerator with a seed.
• qrand() is now a call of generate() from the QRandomGenerator Object
• toTime_t() -> toSecsSinceEpoch()
• QString::SkipEmptyParts -> Qt::SkipEmptyParts
• append(someQstring) -> append(someQstring.toUtf8())`
• QJsonDocument::fromBinaryData() -> QJsonDocument::fromJson()
• QJsonDocument::toBinaryData() -> QJsonDocument::toJson()

Everything works aswell in Qt5. The SkipEmptyParts will not work but I made a preprocessor variable for this:

#if QT_VERSION >= 0x060000
    #define SKIP_EMPTY_PARTS Qt::SkipEmptyParts
#else
    #define SKIP_EMPTY_PARTS QString::SkipEmptyParts
#endif

I did not change the CMakeLists.txt to Qt6, since it is still a generator for Qt5.

PING @wing328 @ravinikam (2017/07) @stkrwork (2017/07) @etherealjoy (2018/02) @MartinDelille (2018/03) @muttleyxd (2019/08)