Prevent the backport action from looping endlessly

[lenaschoenburg]

Description

In a recent PR, the backport action triggered itself, causing an endless loop.

This happened because the backport action is triggered on issue comments that contain the string /backport:
https://github.com/camunda/zeebe/blob/3ff54bfbeba499ad8c317465e043857f1c4872dd/.github/workflows/backport.yml#L1-L21

and on backport failure, it adds a comment that contains that string: #9411 (comment)

Previously, this wasn't an issue. Presumably, this is because we recently switched to using a PAT instead of the generic github-actions bot account: #9426

We should adjust the condition that triggers the backport action to either skip comments that are created by the @backport-action user or by restricting the condition so that the /backport must appear at the end of the comment.

We should also add some warnings upstream so that other users don't run into the same issue: https://github.com/zeebe-io/backport-action.

Once we have fixed this behavior, we should also re-enable the backport workflow: https://github.com/camunda/zeebe/actions/workflows/backport.yml