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 (note: this workflow was checking security-dashboards-plugin in its if condition, so it was never actually running for this repo)

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 70.38%. Comparing base (3d9091a) to head (e555f6e).
⚠️ Report is 3 commits behind head on main.

❌ Your project status has failed because the head coverage (70.38%) is below the target coverage (75.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #907      +/-   ##
==========================================
- Coverage   70.48%   70.38%   -0.10%     
==========================================
  Files          42       42              
  Lines        2060     2060              
  Branches      164      164              
==========================================
- Hits         1452     1450       -2     
- Misses        499      501       +2     
  Partials      109      109              

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[dbwiddis]

LGTM! Clean removal of the manual CHANGELOG process, aligned with the org-wide initiative in opensearch-project/OpenSearch#21071. Good catch that the changelog_verifier.yml was checking security-dashboards-plugin and never actually ran for this repo. The Codecov project check failure is a pre-existing coverage gap, unrelated to this PR (no source code changed).