Release 3.0.0.8

[GianTschingt]

Sets the version to 3.0.0.8 (final) in pom.xml and agent/pom.xml, following the release steps in AGENTS.md. Michael approved the release.

• CHANGELOG: the 3.0.0.5 to 3.0.0.8 snapshot sections are merged into one [3.0.0.8] entry. It adds LDEV-6344, which was missing, and gives the minimum core as 7.1.0.136, the actual luceeCoreVersion (the snapshot notes said 7.1.0.134). 7.1.0.204 final and later satisfy it.
• README, JAVA_AGENT.md, docker example: 3.0.0.4 changes to 3.0.0.8, and the README minimum changes from 7.1.0.132+ to 7.1.0.136+.

Release order:

1. Merge CI: pin Maven 3.9.16 for Central deploy #11 (Maven 3.9.16 pin for the deploy job). Without it, Central rejects the bundle on the current runner image.
2. Merge this PR.
3. Go to Actions → Build Extension → Run workflow on main and tick Deploy to Maven Central. Gian only has read access here, so a maintainer has to start it.
4. After Central has it, I open the PR for 3.0.0.9-SNAPSHOT and create the GitHub release.

Tickets in this release: LDEV-6274, LDEV-6282, LDEV-6309, LDEV-6334, LDEV-6335, LDEV-6337, LDEV-6339, LDEV-6344.