Do not delete branches still used by PRs

[xmo-odoo]

odoo/odoo#237800 / odoo/odoo#237801

When the first PR was closed, the mergebot enqueued the deletion for the branch, and didn't take in account that the branch was still being used by an other PR.

When processing a deletion item, the mergebot should probably check that there is no other PR with the same label falling short of the cutoff.

[xmo-odoo]

NOTA: would probably also make sense to subscribe to delete, and remove the remover if a branch is externally deleted. That way if someone deletes the branch, then recreates it anew, the mergebot should not try deleting the branch again a week later.