[8.19][ML] Add per-PR changelog YAML entries with schema validation (#2920)

[edsavage]

Replace the monolithic CHANGELOG.md with per-PR YAML changelog files in docs/changelog/. Each PR that changes user-visible behaviour adds a small YAML file (<PR_NUMBER>.yaml) with structured metadata (area, type, summary). This eliminates merge conflicts in CHANGELOG.md and simplifies backports.

Includes:

• JSON schema for validating changelog entries
• Python validation script (validate_changelogs.py)
• Python bundler script (bundle_changelogs.py) for release notes
• Gradle tasks: validateChangelogs, bundleChangelogs
• Buildkite CI step (soft-fail during rollout)
• Skip validation via >test, >refactoring, >docs, >build labels

Replaces the ml-cpp-specific changelog schema with the exact Elasticsearch changelog schema so that entries can be consumed directly by the ES release notes pipeline (Phase 2 Option A).

Key changes:

• area enum: ES-wide values (most entries use "Machine Learning")
• type enum: adds breaking-java, known-issue, new-aggregation, security, upgrade
• Adds highlight, breaking, and deprecation sub-objects
• pr/area not required for known-issue and security types
• Validator allows descriptive filenames for entries without a pr
• Bundler handles all new types and entries without pr/area
• AsciiDoc output uses {ml-pull} macros for consistency

Made-with: Cursor

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Backports #2920

[prodsecmachine]

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