feat(release): add changelog entry, GitHub Releases, lastPublished, a…

[fabisev]

Issue #, if available: N/A

Description of changes:

Adds changelog + release-visibility improvements to the Maven Central release
workflow, plus maintainer docs.

.github/workflows/release.yml

• New required changelogEntry input. It's prepended to the module's
  RELEASE.CHANGELOG.md (committed into the version-bump PR) and used as the
  GitHub Release notes. Passed via env, never interpolated into the shell.
• New step to create a GitHub Release on the pushed tag, using the changelog
  entry plus a link to the Maven Central artifact.
• New step to update a lastPublished marker in the POM (see below) on each
  release.

Module POMs (core, events, events-sdk-transformer, log4j2,
serialization, tests)

• Added a <!-- lastPublished: X --> comment under <version> recording the
  last version published to Central, seeded with each module's current latest.
  Informational only (Maven ignores it); auto-updated by the release workflow.

RELEASING.md

• New maintainer doc: how to cut a release, what the workflow does, dry runs,
  and failure handling.

No behavior change to the build itself. POMs validated as well-formed;
release.yml validated as valid YAML.

Target (OCI, Managed Runtime, both): both

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 65.79%. Comparing base (f42fcc3) to head (cf830ff).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #640   +/-   ##
=========================================
  Coverage     65.79%   65.79%           
  Complexity      213      213           
=========================================
  Files            34       34           
  Lines           991      991           
  Branches        143      143           
=========================================
  Hits            652      652           
  Misses          287      287           
  Partials         52       52           

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