mergify: add "mergify skip" label

[0xc0170]

Summary of changes

This is useful if we want to drive PR. There are situations when mergify is not ideal,
and we want to overwrite its rules.

I applied the label for some actions that we can overwrite. The use cases here are when status is being checked, as it does not recognize the status properly we need to overwrite it.

Impact of changes

Migration actions required

Documentation

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[X] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

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[0xc0170]

This should fix situations like this: #12509 (we need to overwrite its rules sometimes)

[adbridge]

Not sure if this will work as separate lines, it says multiple labels should be in a [ ] list....

[0xc0170]

I understand the list in add/remove labels in actions but for conditions?
How to write these:

      - "label!=do not merge"
      - "label!=mergify skip"
      - "label=needs: CI"

      - "label!=['do not merge', mergify skip]"
      - "label=needs: CI"

I believe both should have the same result if are loaded from yaml

[0xc0170]

This code snippet was taken from "ready for merge" rule, and that one was working (considering labels), so as it is it should work. Pitty, I can't test here but first merge the config and then on another PR.

[0xc0170]

Ive checked again, also works in case of release label:

-label~=^(release-version)
 -label~=^(Release review required)

So this will work as it is in the PR

[0xc0170]

CI started