mergify: remove backport automation for non active branches

[v1v]

What does this PR do?

Support only active release branches.

Why is it important?

Avoid backports to non-active releases, such as #31976

[mdelapenya]

Do you think this could be automated at the moment a new branch appears?

[v1v]

Do you think this could be automated at the moment a new branch appears?

I think so, but since the automation is somewhere else I wanted to put the fix so no other backports are done to the non-active release branch.

[elasticmachine]

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• Start Time: 2022-06-24T11:29:07.295+0000

• Duration: 15 min 10 sec

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