[mergify] notify if no backport labels on the PRs targeting the master branch

[v1v]

Motivation/summary

1. Tidy up old branches.
2. Notify PR contributors about the backport policy.

This policy will notify what are the different backport options and tag the PR with the backport-skip.

Test

See elastic/apm-pipeline-library#1281 (comment) and elastic/apm-pipeline-library#1281 (comment)

[botelastic]

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[elasticmachine]

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• Start Time: 2021-09-21T10:25:31.335+0000

• Duration: 16 min 19 sec

• Commit: 5f17376

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[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-v./d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.