Upgrade k8s forgejo-runner from v6.3.1 to v12.7.0 (#251)

## Summary

Completes the `upgrade-k8s-runner` mikado chain. Both prerequisites (workflow validation in Dagger, config review against v12 defaults) were resolved in #250.

- Bump runner image `code.forgejo.org/forgejo/runner:6.3.1` → `12.7.0`
- Update `service-versions.yaml` to track new version
- Mark goal card complete (remove `status: active`)

## Deployment and Testing

After merge:
1. `argocd app sync forgejo-runner`
2. Verify runner registers in Forgejo admin → runners
3. Trigger a test workflow (e.g. `branch-cleanup.yaml` manual dispatch)

Rollback: revert image tag to `6.3.1`, push, sync.
Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/251

docs/how-to/forgejo-runner/upgrade-k8s-runner.md

@@ -1,6 +1,5 @@
 ---
 title: Upgrade K8s Forgejo Runner to v12
-status: active
 requires:
   - validate-workflows-against-v12
   - review-runner-config-v12