Remove manual CHANGELOG.md process

[cwperks]

Description

The CHANGELOG.md file has been a source of merge conflicts and has still resulted in poor quality release notes. Release notes will now be generated from PR and commit metadata instead. See opensearch-project/OpenSearch#21071 for the proposal and opensearch-project/OpenSearch#21083 for the equivalent change in core OpenSearch.

Changes

• Replace CHANGELOG.md content with a redirect to the PR search and release notes tooling docs
• Delete .github/workflows/changelog_verifier.yml
• Remove changelog helper and commit steps from .github/workflows/dependabot_pr.yml, replace with unconditional skip-changelog label

Related Issues

Resolves opensearch-project/OpenSearch#21071

Check List

• Commits are signed per the DCO using --signoff.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.59%. Comparing base (5dbcc0c) to head (8c2d377).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2407   +/-   ##
=======================================
  Coverage   71.59%   71.59%           
=======================================
  Files         101      101           
  Lines        3010     3010           
  Branches      511      529   +18     
=======================================
  Hits         2155     2155           
  Misses        699      699           
  Partials      156      156           

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