Monitoring plugin elasticsearch config deprecations

[jportner]

There are several config settings that are deprecated as of 7.16 and show up in the upgrade assistant:

• elasticsearch.username: elastic
  • [Breaking change] forbid elasticsearch.username: elastic in production #51101
• elasticsearch.username: kibana
  • [Breaking change] kibana user removed and replaced by kibana_system #81680
• elasticsearch.ssl.certificate without elasticsearch.ssl.key and vice versa
  • [Breaking change] Forbid using elasticsearch.ssl.certificate without elasticsearch.ssl.key and vice versa #81746

These are not breaking in 8.0, but we want the deprecations to show in the Upgrade Assistant in 7.16 to encourage users to fix their configs. I updated these deprecation warnings for Core in #115241.

The Monitoring plugin inherits these "root" settings by default, or they can be set separately for Monitoring using the monitoring.ui. prefix.
Unfortunately, while working on the PR linked above, I discovered that any issue with the root settings would surface twice in the Upgrade Assistant -- once from Core, and once from the Monitoring plugin.

To ensure that the Upgrade Assistant experience in 7.16 isn't confusing, I removed these deprecations from the Monitoring plugin (see #115241 (comment)).

The deprecations should be added again in the Monitoring plugin, but they should only show up if the Monitoring plugin is not inheriting the root settings from Core.

kibana/x-pack/plugins/monitoring/server/deprecations.ts

Lines 64 to 69 in b7577b5

	// TODO: Add deprecations for "monitoring.ui.elasticsearch.username: elastic" and "monitoring.ui.elasticsearch.username: kibana".
	// TODO: Add deprecations for using "monitoring.ui.elasticsearch.ssl.certificate" without "monitoring.ui.elasticsearch.ssl.key", and
	// vice versa.
	// ^ These deprecations should only be shown if they are explicitly configured for monitoring -- we should not show Monitoring
	// deprecations for these settings if they are inherited from the Core elasticsearch settings.
	// See the Core implementation: src/core/server/elasticsearch/elasticsearch_config.ts

This is not imperative for the 7.16 release, it can be done in the 8.x timeframe.

[elasticmachine]

Pinging @elastic/stack-monitoring (Team:Monitoring)