[Fleet] Remove short-term solution to detecting ESS cname & URL

[joshdover]

In 7.16 we added a deployment details pop-out on the Unified Integrations app to help users find their public URL for connecting to Elasticsearch.

Because Kibana does not know the public URL for on-prem deployments or ECE deployments, we added a less than ideal solution that matches on ESS's known public cnames to determine if we should show the pop-out or not:

kibana/x-pack/plugins/fleet/public/applications/integrations/components/header/deployment_details.tsx

Lines 29 to 40 in 194bddf

	// If the cname doesn't start with a known prefix, we can't display the flyout.
	// TODO: dover - this is a short term solution, see https://github.com/elastic/kibana/pull/114287#issuecomment-940111026
	if (
	!(
	cname.endsWith('elastic-cloud.com') ||
	cname.endsWith('found.io') ||
	cname.endsWith('found.no') ||
	cname.endsWith('foundit.no')
	)
	) {
	return null;
	}

We should replace this with a dedicated config value passed into the xpack.cloud.* config that includes the public ES URL. Something like publicElasticsearchUrl. Instead of being Cloud-specific, this could also be a generic feature that we could use for on-prem as well. For instance this could be supplied to the Core config key elasticsearch.publicUrl which we could then populate on ESS, ECE, and document for users to configure themselves. cc @elastic/kibana-core

[joshdover]

fixed in #124020