[Fleet] Add cloud native monitoring team as codeowner for Fleet k8's manifests (#170640)

## Summary

Adds @elastic/obs-cloudnative-monitoring as a code owner for the Fleet
K8's manifest files.

These files are updated via automation in the
https://github.com/elastic/elastic-agent repo. However, the automation
currently generates empty diffs and general noise for the Fleet team,
who are pinged for code owner review.

Here are the PR's we've closed due to empty diffs or inaccurate
manifests in the last few months:
- https://github.com/elastic/kibana/pull/170616
- https://github.com/elastic/kibana/pull/170544
- https://github.com/elastic/kibana/pull/169006
- https://github.com/elastic/kibana/pull/168715
- https://github.com/elastic/kibana/pull/166590
- https://github.com/elastic/kibana/pull/166526
- https://github.com/elastic/kibana/pull/165606
- https://github.com/elastic/kibana/pull/165471

Adding the cloud native monitoring team as code owners directly should
remove the need for Fleet to manually ping somebody when there's issues
with these PR's.

.github/CODEOWNERS

@@ -952,6 +952,7 @@ x-pack/plugins/infra/server/lib/alerting @elastic/actionable-observability
 /x-pack/test/fleet_cypress @elastic/fleet
 /x-pack/test/fleet_functional @elastic/fleet
 /src/dev/build/tasks/bundle_fleet_packages.ts @elastic/fleet @elastic/kibana-operations
+/x-pack/plugins/fleet/server/services/elastic_agent_manifest.ts @elastic/fleet @elastic/obs-cloudnative-monitoring
 
 # APM
 /x-pack/test/functional/apps/apm/  @elastic/apm-ui