ci: validate unreleased Flint Checkstyle support

[zeitlinger]

Summary

• pin Flint to the unreleased Checkstyle implementation from feat: add standalone Checkstyle linting grafana/flint#435
• install standalone Checkstyle 13.8.0 through the new Aqua package from feat: add checkstyle/checkstyle aquaproj/aqua-registry#57625
• remove the Maven Checkstyle plugin and its obsolete skip settings so Checkstyle runs only once, through Flint
• preserve the repository's previous main-source scope with narrow suppressions for test and archived Java sources
• suppress VariableDeclarationUsageDistance for examples, where setup declarations intentionally precede simulated work

This is a pre-release consumer validation PR. It should be reverted to a released aqua:grafana/flint pin once the Flint release is available.

Validation

Using the Aqua registry branch from aquaproj/aqua-registry#57625:

• mise exec -- flint run --full checkstyle
• mise run lint:fix
• mise run build

[zeitlinger]

This now removes the Maven Checkstyle plugin to avoid running Checkstyle twice. The PR uses the Aqua package from aquaproj/aqua-registry#57625; CI installation depends on that registry PR merging.