ci: upload gem from GitLab release stage

[brettlangdon]

What does this PR do?
Automates Ruby gem publishing by uploading from GitLab pipeline.

Motivation:
Remove manual step of uploading the gem on release.

Change log entry

None.

Additional Notes:

This is modeled after the dd-trace-java and dd-trace-py processes which assume that a Git tag/GitHub Release are created first (that is the trigger to "do a release"). Rather than existing Ruby process which first builds/checks tests, then uploads the Gem, and then finally creates the GitHub Release. We can adjust this workflow, but wanted to give us a place to start, aligning with others was simplest.

Since this is done in a final release stage, it does mean that the GitLab pipeline needs to fully run and complete before the Gem will be uploaded. Any checks/tests added to GitLab will become a blocker to the Gem being uploaded.

This process also uses the built Gem that we already do in the pipeline and use in the OCI/lib-injection artifacts.

We have not added any additional checks on GitHub commit status, we can/should do that. I'd love to add to the one pipeline so it is implemented consistently for all repos.

I have not yet added the necessary RubyGem token to the process.

How to test the change?

[github-actions]

Thank you for updating Change log entry section 👏

^{Visited at: 2025-01-24 15:43:14 UTC}

[datadog-datadog-prod-us1]

Datadog Report

Branch report: brettlangdon/gitlab.release
Commit report: 6a99285
Test service: dd-trace-rb

✅ 0 Failed, 22100 Passed, 1476 Skipped, 5m 15.55s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-01-24 16:00:20

Comparing candidate commit 3c293b6 in PR branch brettlangdon/gitlab.release with baseline commit 8818eb8 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 31 metrics, 2 unstable metrics.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.71%. Comparing base (8818eb8) to head (6a99285).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4322      +/-   ##
==========================================
- Coverage   97.72%   97.71%   -0.01%     
==========================================
  Files        1365     1365              
  Lines       82780    82780              
  Branches     4206     4206              
==========================================
- Hits        80897    80889       -8     
- Misses       1883     1891       +8     

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