ci/mergify: add merge rule for backport to 0.44 (backport #3862)

[mergify]

Describe your changes

Allows to use "merge" label with backported PRs

Checklist before merging

• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
  • This will require 2 reviewers to approve the changes
• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
  • Relevant PR if applies:
• If this PR affects services such as namada-indexer or namada-masp-indexer, a corresponding PR is opened in that repo
  • Relevant PR if applies:

---

This is an automatic backport of pull request #3862 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of b0ae7bc has failed:

On branch mergify/bp/0.44.0/pr-3862
Your branch is up to date with 'origin/0.44.0'.

You are currently cherry-picking commit b0ae7bcc9.
  (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 <file>..." to mark resolution)
	both modified:   .mergify.yml

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.04%. Comparing base (0eecd3b) to head (a164b71).
Report is 3 commits behind head on 0.44.0.

Additional details and impacted files

@@           Coverage Diff           @@
##           0.44.0    #3863   +/-   ##
=======================================
  Coverage   73.03%   73.04%           
=======================================
  Files         340      340           
  Lines      104816   104816           
=======================================
+ Hits        76552    76562   +10     
+ Misses      28264    28254   -10     

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