mergify: add backport strategy (backport #3849)

[mergify]

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• If this PR has some consensus breaking changes, I added the corresponding breaking:: labels
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• If this PR requires changes to the docs or specs, a corresponding PR is opened in the namada-docs repo
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This is an automatic backport of pull request #3849 done by [Mergify](https://mergify.com).

[tzemanovic]

I've backported this one manually in #3865