[mergify] add backport policy and default label

[v1v]

Motivation/summary

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.

We can think about enforcing this rule to do not allow merges but for the time being, what do you think?

Test

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

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• Start Time: 2021-09-20T11:32:12.233+0000

• Duration: 39 min 56 sec

• Commit: ef21ebe

Test stats 🧪

Test	Results
Failed	0
Passed	6101
Skipped	14
Total	6115

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[axw]

Nice, thanks @v1v

We can think about enforcing this rule to do not allow merges but for the time being, what do you think?

I think this is a good step. If the nudge isn't enough, then we can come back to blocking merges.

[v1v]

There is a missing bit regarding skipping the backports validation