ci: adopt release-please for automated versioning and Maven Central publishing

[emaarco]

What

Adopt release-please for Conventional-Commit-driven releases and move Maven Central publishing into the release pipeline. Trunk becomes develop; master is retired.

What changes

• Releases become automatic: feat:/fix: on develop → release-please keeps a rolling Release PR (bumps all 3 poms + CHANGELOG.md); merging it cuts the GitHub Release + tag and deploys to Maven Central in the same run.
• Removed: push-to-master deploy (master.yml) and milestone-based release notes (release-notes.yml).
• Versioning: SemVer, 3-part (e.g. 1.8.0), derived from commit types. poms move 1.8-SNAPSHOT → 1.7.0 baseline that release-please owns.
• CHANGELOG.md added, seeded with the full history of the 25 existing GitHub Releases (which stay untouched); release-please prepends new versions.
• PR titles must be Conventional Commits (squash-merge) — enforced by pr-title.yml; Dependabot commits are now chore-prefixed.
• GPG / Sonatype publishing secrets are unchanged.

Before merge (why draft)

Requires a GitHub App token via vars.RELEASE_PLEASE_APP_CLIENT_ID + secrets.RELEASE_PLEASE_APP_PRIVATE_KEY — the default GITHUB_TOKEN cannot trigger CI on the Release PR. The release job fails at the token step until the App is configured.

Verified locally: mvnw validate → BUILD SUCCESS; all YAML/JSON valid.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.65%. Comparing base (d4d1bd8) to head (970281f).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #301   +/-   ##
==========================================
  Coverage      50.65%   50.65%           
  Complexity        80       80           
==========================================
  Files             40       40           
  Lines            458      458           
  Branches           9        9           
==========================================
  Hits             232      232           
  Misses           221      221           
  Partials           5        5           

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[emaarco]

@zambrovski - drafted, how a release would look like with release-please. Would then:

• creates or updates PR when stuff is merged on main
• updates the changelog then accordingly
• updates the release-version as well - based on semantic commits.
• creates a tag when PR gets merged
• starts the release-workflow for maven when tag got created and releases based on tag