[Qt5][C++] Removed deprecated functions to support Qt6

[basyskom-dege]

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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)

[etherealjoy]

I think a current LTS debian distribution should build off the shelf.

[basyskom-dege]

Okay I have to check for the compatibility of those functions. Will do a new commit on Monday to address the compatibility issues here. Is there a min. required version for Qt? Otherwise i would take the oldest LTS version ( 5.9 LTS )

[basyskom-dege]

I added version checks for the random functions. I kept the changes to the toTime_t() function even if its not available in Qt 5.8 but since the oldest LTS is 5.9, I think thats okay. Do you think I should consider supporting older versions? If yes, whats your oldes version you want to support?
EDIT: since the build failed, I added checks for the version for all functions that I changed, except for toUft8().

[etherealjoy]

The current build failure, seems unrelated to the changes.

[MartinDelille]

Since these changes solve annoying deprecated warning with Qt 5.15, I would rather check versus 5.15 instead of 6.0. What do you think?

[basyskom-dege]

Since these changes solve annoying deprecated warning with Qt 5.15, I would rather check versus 5.15 instead of 6.0. What do you think?

I wouldnt mind that. Those warnings are numerous thats true.

[wing328]

Travis CI tests passed via https://travis-ci.org/github/OpenAPITools/openapi-generator/builds/756044989