[repo] Release notes automation

[CodeBlanch]

Changes

• Adds automation for updating RELEASENOTES.md when performing stable core- releases

Details

RELEASENOTES.md should be updated before creating the release tag because a permalink is included in packages. To help make sure this is done correctly the bot/automation will now detect missing content and remind users to add something. It also now has a command to help do this right from the PR discussion.

Test runs

• Stable release run without content: [release] Prepare release core-1.12.0 CodeBlanchOrg/opentelemetry-dotnet#66
  Bot reminds user to add release note content and then has a command to help.

• Stable release run with content: [release] Prepare release core-1.11.0 CodeBlanchOrg/opentelemetry-dotnet#65
  Bot doesn't nag to do anything but the command is still available.

• Prerelease run: [release] Prepare release core-1.12.0-beta.1 CodeBlanchOrg/opentelemetry-dotnet#64
  No reminder, updates ignored for prerelease builds.

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)

[Kielek]

I like the idea,

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.38%. Comparing base (75d947c) to head (3605cb6).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6070   +/-   ##
=======================================
  Coverage   86.38%   86.38%           
=======================================
  Files         257      257           
  Lines       11679    11679           
=======================================
  Hits        10089    10089           
  Misses       1590     1590