contributing: Optimize commands, text in release procedure (G83)

[neteler]

Partial sync to main, following #2414:

• Add cron job update
• Restructure updates to website and wiki
• Remove Launchpad notes (apparently not used, at least not by the release manager)
• Change of procedure for the mailing list announcements
• Improves precision in the text

Note that there are still differences in the initial git sync part: also to be synced?

[wenzeslaus]

[...] differences in the initial git sync part: also to be synced?

I would say so. The files should be almost identical except for version numbers and where we are changing the procedure (I don't recall that we do). I tried to create a diff online, but it didn't work, so local Meld is probably the best (this is what I got - I don't know why it shows 8.2 stuff in red. I expected 8.3).