[ci] Quick retry for backports

[delanni]

The current method to retry an automatic backport is through removing and re-adding a label. It's not a straightforward and convenient way.

We could research what level of effort it would take to move a backport-retry function to a [ ] checkbox-activated function, or any other, more convenient method to make it easier for developers to deal with the reminders.

See: https://elastic.slack.com/archives/C5UDAFZQU/p1741862229286759

[elasticmachine]

Pinging @elastic/kibana-operations (Team:Operations)