Remove Java-specific CHANGELOG; use monorepo's release-changelog agent for java/v* and rust/v* scoped tags.

Address [comments](#1369 (comment)) from @stephentoub .

.github/scripts/release/update-changelog.sh (deleted): Custom awk-based script that maintained java/CHANGELOG.md is no longer needed.
.github/scripts/release/test-update-changelog.sh (deleted): Tests for the above script.
.github/workflows/java-publish-maven.yml: Remove update-changelog.sh call and CHANGELOG.md from git-add; add "Trigger changelog generation" step that invokes release-changelog.lock.yml with the java/v* tag.
.github/workflows/release-changelog.md: Surgical additions to handle language-prefixed tags (java/v*, rust/v*) — scoped previous-tag lookup, scoped file filtering, scoped code snippets, and scoped heading format in CHANGELOG.md.
.github/workflows/release-changelog.lock.yml: Recompiled from release-changelog.md via `gh aw compile`.
java/CHANGELOG.md (deleted): No longer maintaining a Java-specific changelog; releases flow through the root CHANGELOG.md via the release-changelog agent.

Author: edburns