Add post-release workflow step to create a PR with version bump

[y9v]

What does this PR do?
This PR adds a Github Actions workflow called Bump datadog gem version that generates a PR with the next prerelease version. This workflow is triggered manually.

This is done using existing rake version:bump task that increases the minor version and sets the tiny version to zero.

Motivation:
We want to have the next prerelease version in our version.rb file to simplify system-tests setup.

Change log entry
None. This change is internal.

Additional Notes:
This should not be merged yet, since we will need to do some changes to fast_castle to not break our release process.

How to test the change?
Here is the generated PR: #4797 (commit message has been fixed)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.54%. Comparing base (2a76c95) to head (8b032b3).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4765      +/-   ##
==========================================
- Coverage   97.55%   97.54%   -0.01%     
==========================================
  Files        1484     1484              
  Lines       88722    88729       +7     
  Branches     4606     4606              
==========================================
  Hits        86553    86553              
- Misses       2169     2176       +7     

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[datadog-datadog-prod-us1]

Datadog Report

Branch report: generate-pr-to-update-version-in-publish-workflow
Commit report: 9f9191a
Test service: dd-trace-rb

✅ 0 Failed, 21794 Passed, 1296 Skipped, 4m 54.14s Total Time

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-07-15 10:16:28

Comparing candidate commit 8b032b3 in PR branch generate-pr-to-update-version-in-publish-workflow with baseline commit 2a76c95 in branch master.

Found 0 performance improvements and 4 performance regressions! Performance is the same for 43 metrics, 2 unstable metrics.

scenario:error - error tracking with http - all

• 🟥 throughput [-151.182op/s; -147.650op/s] or [-96.244%; -93.995%]

scenario:error - error tracking with http - third_party only

• 🟥 throughput [-149.445op/s; -147.074op/s] or [-95.911%; -94.389%]

scenario:error - error tracking with http - user code only

• 🟥 throughput [-151.008op/s; -148.121op/s] or [-96.133%; -94.295%]

scenario:error - without error tracking with http

• 🟥 throughput [-149.853op/s; -146.188op/s] or [-96.218%; -93.865%]

[Strech]

The only question is fast_castle, will be there a clash on release?

[y9v]

@Strech yes, there will be, and there is one more problem - we need to know the previous released version, and with this change we won't have it. So there will be another PR for fast_castle this week