Replace _xpack/monitoring/* with _monitoring/* ES API calls

[ycombinator]

Starting 7.0, the _xpack/monitoring/* API endpoints in Elasticsearch will be deprecated in favor of monitoring/* API endpoints. We should update any references to the deprecated endpoints in the Kibana codebase and replace them with the new ones.

[elasticmachine]

Pinging @elastic/kibana-monitoring

[ycombinator]

Re-pinging @elastic/stack-monitoring

[chrisronline]

This is handled by #29465

[chrisronline]

Closed by #29465