Add release automation

[mdellweg]

• Create release branch from main and bump minor version
• Release from a release branch and bump patch version

[noissue]

Review Checklist:

• An issue is properly linked. [feature and bugfix only]
• Tests are present or not feasible.
• Commits are split in a logical way (not historically).

[ggainey]

Automation is good - let's see how it flies in practice!

[patchback]

Backport to 0.20: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply b138270 on top of patchback/backports/0.20/b138270d9085394aa7f1cd751fb0a7ddf2c0bcaa/pr-718

Backporting merged PR #718 into main

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/pulp/pulp-cli.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/0.20/b138270d9085394aa7f1cd751fb0a7ddf2c0bcaa/pr-718 upstream/0.20

4. Now, cherry-pick PR Add release automation #718 contents into that branch:

   $ git cherry-pick -x b138270d9085394aa7f1cd751fb0a7ddf2c0bcaa

   If it'll yell at you with something like fatal: Commit b138270d9085394aa7f1cd751fb0a7ddf2c0bcaa is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x b138270d9085394aa7f1cd751fb0a7ddf2c0bcaa

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Add release automation #718 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/0.20/b138270d9085394aa7f1cd751fb0a7ddf2c0bcaa/pr-718

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
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