chore: use only mergify for backports

[v1v]

After enabling mergify for the backports, let's remove the former approach

[mergify]

This pull request does not have a backport label. Could you fix it @v1v? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.
• If no backport is necessary, please add the backport-skip label

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 5e345a0

[mashhurs]

LGTM!

[v1v]

@mashhurs , I don't have write accss, so I cannot merge it, do you mind merging this PR?