mergify: add backport strategy

[Fraccaman]

Describe your changes

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:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.03%. Comparing base (36ca7c3) to head (63ae527).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3849      +/-   ##
==========================================
- Coverage   73.04%   73.03%   -0.01%     
==========================================
  Files         340      340              
  Lines      104816   104816              
==========================================
- Hits        76559    76557       -2     
- Misses      28257    28259       +2     

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