chore: categorize generated release notes by label

[zimeg]

This pull request adds a .github/release.yml that groups automatically generated release notes by label, matching the Python SDK, Bolt for Python, and the Slack CLI.

• Pull requests labeled enhancement, bug, docs, build, code health, or security are listed under their own headings. Everything else falls under "Other changes".
• The maintainers guide now uses the "Generate release notes" button when drafting a GitHub Release, in place of the git log command.

Testing

• Open https://github.com/slackapi/java-slack-sdk/releases/new, choose the latest tag, and click "Generate release notes" after this merges to confirm changes are grouped by label.

Category (place an x in each of the [ ])

• bolt (Bolt for Java)
• bolt-{sub modules} (Bolt for Java - optional modules)
• slack-api-client (Slack API Clients)
• slack-api-model (Slack API Data Models)
• slack-api-*-kotlin-extension (Kotlin Extensions for Slack API Clients)
• slack-app-backend (The primitive layer of Bolt for Java)

Requirements

Please read the Contributing guidelines and Code of Conduct before creating this issue or pull request. By submitting, you agree to those rules.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.90%. Comparing base (43fad0e) to head (428b786).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1652      +/-   ##
============================================
- Coverage     72.90%   72.90%   -0.01%     
+ Complexity     4594     4593       -1     
============================================
  Files           483      483              
  Lines         14565    14565              
  Branches       1520     1520              
============================================
- Hits          10619    10618       -1     
- Misses         3043     3044       +1     
  Partials        903      903              

Flag	Coverage Δ
jdk-14	72.90% <ø> (-0.01%)	⬇️

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