Clean up Stale Branches for simplicity

[nerrixde]

... and the "Forums" link in readme is outdated

[vinckr]

Thanks a lot for heads up !
I updated the README to point to github discussions instead of the old forum.
As for the stale branches I think @zepatrik has to look at that.

[zepatrik]

Cleaned up all that had a closed PR or came from automation. ✔️