Use a single file for migration + release notes (backport #324)

[mergify]

we decided to show only the notes for the respective version in the docs, instead of all incremental distro upgrades

migration

release

---

This is an automatic backport of pull request #324 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 698e41f has failed:

On branch mergify/bp/iron/pr-324
Your branch is up to date with 'origin/iron'.

You are currently cherry-picking commit 698e41f.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	both modified:   doc/migration/migration.rst
	deleted by us:   doc/release_notes/release_notes.rst

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally