Add action to label and close stalled PRs

[andrross]

Description

This action labels PRs as "stalled" after 30 days, then closes them 7 days later if no follow up.

The following choices here were all fairly arbitrary and open for discussion:

• When to run: every morning, Pacific Time
• What label to use: stalled, because it already exists in the repo but appears to be unused
• Inactivity period to label as stalled: 30 days
• Period after stalled before closing: 7 days

Related Issues

Resolves #8093

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff
• Commit changes are listed out in CHANGELOG.md file (See: Changelog)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: SUCCESS ✅
• URL: https://build.ci.opensearch.org/job/gradle-check/18025/
• CommitID: 99326fa

[codecov]

Codecov Report

Merging #8178 (117618c) into main (5dce570) will decrease coverage by 0.20%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main    #8178      +/-   ##
============================================
- Coverage     70.93%   70.73%   -0.20%     
+ Complexity    56633    56555      -78     
============================================
  Files          4719     4719              
  Lines        267555   267555              
  Branches      39206    39206              
============================================
- Hits         189778   189258     -520     
- Misses        61721    62345     +624     
+ Partials      16056    15952     -104     

see 482 files with indirect coverage changes

[github-actions]

Gradle Check (Jenkins) Run Completed with:

• RESULT: UNSTABLE ❕
• TEST FAILURES:

      2 org.opensearch.index.shard.RemoteStoreRefreshListenerTests.classMethod
      1 org.opensearch.search.backpressure.SearchBackpressureIT.testSearchShardTaskCancellationWithHighCpu
      1 org.opensearch.indices.replication.SegmentReplicationIT.testScrollCreatedOnReplica
      1 org.opensearch.index.shard.RemoteStoreRefreshListenerTests.testRefreshSuccessAfterFailureInFirstAttemptAfterSnapshotAndMetadataUpload

• URL: https://build.ci.opensearch.org/job/gradle-check/18048/
• CommitID: 117618c
  Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.