GitHub Actions: Disable Ubuntu 18.04 CI build

[Holzhaus]

Ubuntu 18.04 will not be supported for 2.4.

This is a prerequisite for PR #3038. Due to the old Qt 5.9 in Ubuntu the tests fail because const (ES6/7) does not work (Qt < 5.12 only supports ES3):

warning [0x56218251da90] LINT: "/home/runner/work/mixxx/mixxx/res/controllers/Traktor Kontrol Z2.hid.xml" has no forum link.
warning [0x56218251da90] LINT: "/home/runner/work/mixxx/mixxx/res/controllers/Traktor Kontrol Z2.hid.xml" has no wiki link.
warning [0x56218251da90] QFileSystemWatcher::removePaths: list is empty
warning [0x56218251da90] ControllerScriptHandlerBase: "Uncaught exception at line 12 in file /home/runner/work/mixxx/mixxx/res/controllers/Traktor-Kontrol-Z2-hid-scripts.js.\n\nException:\n  SyntaxError: Syntax error"
critical [0x56218251da90] DEBUG ASSERT: "m_pJSEngine" in function bool ControllerScriptEngineLegacy::callFunctionOnObjects(const QList<QString>&, const QString&, const QJSValueList&, bool) at /home/runner/work/mixxx/mixxx/src/controllers/scripting/legacy/controllerscriptenginelegacy.cpp:29

[uklotzde]

About time to move on! LGTM

[uklotzde]

After arriving in December 2018 we can finally get rid of some workarounds, see QT_VERSION_CHECK.

Especially using QMetaObject::invokeMethod() becomes easier and safer.