Migrate plugin build to Bazel modules

Switch the plugin build from the legacy WORKSPACE-based setup to Bazel
modules (Bzlmod) to align with Gerrit core.

This replaces the previous tools/bzl:plugin.bzl-based configuration with
gerrit_plugin() and gerrit_plugin_tests() from bazlets and moves
external Maven dependencies to MODULE.bazel via rules_jvm_external.

Runtime dependencies are now provided via the javamelody_plugin_deps
repo exposed from the module extension and referenced from the plugin
BUILD file.

Add runtime_jars_allowlist_test and runtime_jars_overlap_test to track
third-party JARs packaged into the plugin and to detect classpath
overlaps with Gerrit core when building in-tree.

The overlap check is enabled only in Gerrit-tree builds via the
in_gerrit_tree flag and skipped in standalone builds.

Update Bazel version and Java toolchain settings accordingly.

Change-Id: I1015ec5ee3867536692d52421a9906d82a700ebb