Android: Qt6 compatibility

**What is the current behaviour and why should it be changed?**

Jamulus will fail to build with Qt6 even when #2299 is merged as it uses androidextras for permission handling, which is no longer available.

**Describe possible approaches**

1. Stay on unsupported Qt 5.15.2
2. Use the private implementation when building against Qt6 as described in the relevant [Qt ticket](https://bugreports.qt.io/browse/QTBUG-84382?focusedCommentId=566019&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-566019)

I'd go with (2), although it is not nice either.

**Has this feature been discussed and generally agreed?**

No.