Mergify: fix release version label

[0xc0170]

Check if release review is not applied for merged PR. If not, we use "release version missing" label.

Summary of changes

Adding new label to cover when PR is not versioned. Fixing the current behavior.
Note @ARMmbed/mbed-os-maintainers Before merging, please make sure PR is either "Release review required" or "release-version" to mark versions correctly.

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]

Tested with mergify simulator on PR #12321 (has review release label):

#### Rule: release version is a must for merged PRs (label, comment)
- [X] `merged`
- [X] `-label~=^(release-version)`
- [ ] `-label~=^(Release review required)`

and Pr without any label for release (review or version):

#### Rule: release version is a must for merged PRs (label, comment)
- [X] `merged`
- [X] `-label~=^(release-version)`
- [X] `-label~=^(Release review required)`

[ciarmcom]

@0xc0170, thank you for your changes.
@ARMmbed/mbed-os-maintainers please review.

[adbridge]

LGTM

[0xc0170]

CI started