Add support for FFmpeg 7.0

We need to make sure all security critical 3rd party modules (https://wiki.qt.io/Third_Party_Code_in_Qt) are the latest one before we can release.

It seems that ffmpeg needs an update; we are using n6.1 but n7.0 is already available:
https://github.com/FFmpeg/FFmpeg/tags

FFMPEG is only supported by Chromium 125.

Tasks:

• Update provisioning to build FFmpeg 7.0
• Fix issues found in Qt Webengine when compiling against new FFmpeg
• Test that no regressions are introduced
• Run all auto-tests on local computer for relevant platforms.
• Update documentation (multiple places) to refer to the right version.