Simplify branching model

We de-facto use the gitflow branching model at https://nvie.com/posts/a-successful-git-branching-model/
BUT the "master/develop" branches are making double duty and serve not purpose.
In practice all the code merged in develop is immediately usable in production.
Therefore I suggest we switch to a simpler branching model which is the de facto model we use today:
1. one single default branch called "main" where only merges are done
2. features are pushed in other branches and merged when fully ready in "main"
3. now and then we tag and cut a formal release from "main"
4. exceptionally and likely never we can branch from a tag to backport a hotfix just there