[HOLD] feat: skip check-versions if a label is applied

[pepopowitz]

HOLD: Test cases have not been proven, and I'm shifting priorities for the next few weeks.

When I get back to this, the remaining work is to open PRs into this one, to validate the test cases listed below.

Description

Adds the ability to skip the check-versions workflow, by adding a check-versions:skip label to a PR.

• If the check-versions:skip label is found on a PR at the time of check-versions running, it will skip the check entirely.
• When the check-versions:skip label is applied,

### Test cases
- [ ] When the label is not applied to a new PR with no missing versions, the PR checks for missing versions and does not post a comment.
- [ ] When the label is added to a PR which has no missing version warnings, nothing happens.
- [ ] When more changes are added to a PR with the label already added, missing versions are *not* checked.
- [ ] When the label is applied to a new PR, which contains missing versions, missing versions are *not* checked, and no comment is added.
- [ ] When the label is removed from a PR, and missing version changes are added, the PR checks for missing versions and a comment appears.
- [ ] When a PR contains missing versions and a comment, and the label is added, the comment is updated to acknowledge that you're opting out of version checking.
- [ ] When a PR comes from a fork, and it contains missing versions and a comment, and then the label is added, the comment is updated to acknowledge that you're opting out.
- [ ] When a PR kicks off check-versions, but then the label is added while the check is running, the winning comment in the end is the one that says you opted out of version checking. 

When should this change go live?

• This change is not yet live and should not be merged until {ADD_DATE} (apply hold label or convert to draft PR)?
• There is no urgency with this change.
• This change or page is part of a marketing blog, conference talk, or something else on a schedule.
• This functionality is already available but undocumented.
• This is a bug fix or security concern.

PR Checklist

• I have added changes to the relevant /versioned_docs directory, or they are not for an already released version.
• I have added changes to the main /docs directory (aka /next/), or they are not for future versions.
• My changes require an Engineering review, and I've assigned an engineering manager or tech lead as a reviewer, or my changes do not require an Engineering review.
• My changes require a technical writer review, and I've assigned @christinaausley as a reviewer, or my changes do not require a technical writer review.

[github-actions]

👋 🤖 ✅ Looks like the changes were ported across versions, nice job! 🎉

You can read more about the versioning within our docs in our documentation guidelines.

[christinaausley]

@pepopowitz Do you plan to get to this before you head out or shall we close this PR? 😿

[pepopowitz]

Closing!