move releases such as `dev-23a` and `23b` to `releases/dev-23a` and `releases/23b`

This will make it easier to search for releases: we can look for everything starting with releases.

At the moment, the branch names mix merge request from development with the released version.

The naming of the releases as releases/VERSION follows what spack does:

git branch -a
...
  remotes/origin/releases/v0.18
  remotes/origin/releases/v0.19
  remotes/origin/releases/v0.20
...