Bump version to 3.0.0-SNAPSHOT for XP 8

[rymsha]

Summary

Branches out the XP 7 maintenance line at 2.x (cd10776 — the post-v2.1.1 SNAPSHOT bump) and configures master so CI treats both master and 2.x as release branches going forward.

• .github/workflows/enonic-gradle.yml: add releaseBranch: block listing master + 2.x so build-and-publish recognises both as release branches.
• .github/workflows/enonic-docgen.yml: also fire docgen on push to 2.x.
• docs/versions.json: declare 2.x as stable / latest and master as next, matching the app-booster reference.

gradle.properties already declares version=3.0.0-SNAPSHOT, so no version bump is needed on master.

A companion PR will land on 2.x itself to add the same releaseBranch: block (the release-tools action reads releaseBranch: from each branch's own workflow file).

Test plan

• CI on this PR passes (gradle build green).
• After merge, a push to master is still classified as a release branch (build.outputs.release == 'true').
• After the companion 2.x PR merges, a push to 2.x is also classified as a release branch.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.85%. Comparing base (297eb41) to head (ac73b24).

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #169   +/-   ##
=========================================
  Coverage     78.85%   78.85%           
  Complexity      128      128           
=========================================
  Files            25       25           
  Lines           402      402           
  Branches         25       25           
=========================================
  Hits            317      317           
  Misses           65       65           
  Partials         20       20           

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