Automatically setting Fixed in release on PR merge

I would love to see theese processes formalized and maintained somewhere. I do not have enough experience how to do so for being really helpful, but some graphviz, or even just an draw.io process chart would be nice to have.

Anyway, I love automating that :+1: I feel this really necessary as some maintainers easily forget (for example @ezr-ondrej is really slopy one! :sweat_smile:)