Add stable2412 to target_branches for command-backport.yml

[bkontur]

The backport bot opens PR for A4-needs-backport only for stable2407 stable2409, but we have already stable2412.

The question is, when should we append a new stable* branch here? Should it be done when a new stable* branch is created? Can we automate this process somehow?

[bkontur]

I had to create backport PR to stable2412 manually, but then the bot complains: #6649 (comment)

  This pull request is amending an existing release. Please proceed with extreme caution,
  as to not impact downstream teams that rely on the stability of it. Some things to consider:
  - Backports are only for 'patch' or 'minor' changes. No 'major' or other breaking change.
  - Should be a legit *fix* for some  bug, not adding tons of new features.
  - Must either be already audited or not need an audit.

  <details><summary><i>Emergency Bypass</i></summary>
  <p>

  If you really need to bypass this check: add <code>validate: false</code> to each crate
  in the Prdoc where a breaking change is introduced. This will release a new major
  version of that crate and all its reverse dependencies and basically break the release.

  </p>
  </details>