[Reporting] Switch to the new Elasticsearch client

[tsullivan]

Reporting uses the legacy interface of the Elasticsearch service in a few places:

x-pack/plugins/reporting/server/routes/diagnostic/config.ts:      const { callAsInternalUser } = elasticsearch.legacy.client;
x-pack/plugins/reporting/server/routes/lib/jobs_query.ts:  const { callAsInternalUser } = elasticsearch.legacy.client;
x-pack/plugins/reporting/server/export_types/csv/execute_job.ts:    const { callAsCurrentUser } = elasticsearch.legacy.client.asScoped(fakeRequest);
x-pack/plugins/reporting/server/lib/store/store.ts:  private client: ElasticsearchServiceSetup['legacy']['client'];

These areas need to be updated to use the new Elasticsearch JS client. There already is an async method of the ReportingCore class that provides it (getEsClient) but it might need to be converted to a synchronous method (use the service from the PluginSetup API instead of PluginStart?) to serve the new areas.

After that's done, we need to update Reporting status on the Meta issue: #83910

Documentation: https://www.elastic.co/guide/en/kibana/current/migrating-legacy-plugins-examples.html#_elasticsearch_client

[elasticmachine]

Pinging @elastic/kibana-app-services (Team:AppServices)

[elasticmachine]

Pinging @elastic/kibana-reporting-services (Team:Reporting Services)