chore: configure Renovate for Gradle library updates

[arnaualcazar]

Adding some rules to make renovate bump libraries

Summary

Adds .github/renovate.json5 so the self-hosted Renovate (seqeralabs/renovate-runner) updates Gradle dependencies, not only GitHub Actions. Modelled on platform's .github/renovate.json5.

• enabledManagers: ["gradle", "github-actions"] opts the repo into Gradle updates; the runner's defaults (bot identity, chore: commits, 5-day cooldown, weekly actions window, no actions majors) still apply.
• Scans every module build.gradle, the buildSrc convention plugins, settings.gradle and gradle.properties (versions are hard-coded here, unlike platform's properties-only setup). micronautPlatformVersion is detected via the micronaut { version … } block.
• Major bumps wait for approval on the Dependency Dashboard (migrations; non-Micronaut libs still target Java 17).
• Grouped PRs for lockstep families: Micronaut core/test/platform, Groovy, Spock, Logback, SLF4J, Moshi, Jackson.
• Spock keeps its -groovy-X.Y suffix fixed, so it follows the Groovy line in use.

Dependency bumps don't publish anything: a module is only released when its VERSION is bumped in a [release] PR.

Validation

• renovate-config-validator --strict passes.
• Local renovate --platform=local --dry-run=lookup run: ~15 non-major PRs expected (e.g. Micronaut 4.10.31 group, Groovy 4.0.33, Spock 2.4-groovy-4.0, Logback 1.6.5, Jackson 2.22); majors (Micronaut 5, Groovy 6, Jedis 8, Testcontainers 2, micronaut-cache 6) go to the dashboard for approval.