[Meta] Telemetry & Usage Collection Tech-debt items

[afharo]

@afharo Would it make sense to open a meta issue for general Telemetry-related cleanup tasks we feel are needed before 8.0? To me, it would be a nice way to track all the things that we should be doing before then.

Originally posted by @TinaHeiligers in #91464 (comment)

This issue is to track all the APIs and pieces of logic that are maintained in the Telemetry & Usage Collection plugins but we no longer use, or things we did one way because of some limitations back there, but we're OK to do it better now:

• [Application Usage] Better SO management ([Application Usage] Better SO management #87840)
• Remove __LEGACY.appChanged ([Application Usage] Remove deprecated & unused legacy.appChanged API #91464)
• Remove UI Metric's unused APIs ([Usage Collection] Remove unused UI Metric APIs #91620)
• Remove unused applicationUsageTracker ([Usage Collection] Remove unused applicationUsageTracker from Public API #92670)
• Add telemetry plugins to the core eslint rules ([chore] Add telemetry plugins to the core eslint rules #88816)
• Remove use of any ([Telemetry] Remove uses of any in the Telemetry & Usage Collection plugins and libraries #93183)
• Use import type and export type where possible ([Telemetry] Enforce use of import type and export type #93184)
• Merge telemetry_management_section into the telemetry plugin (could be done as part of [Telemetry] Telemetry Advanced settings section should use the regular UI registration API #56498)
• Remove telemetry_collection_manager in favour of a better X-Pack extension API in the telemetry plugin (Remove telemetry_collection_manager in favour of a better X-Pack extension API in the telemetry plugin #93185)
• [Telemetry] Move all collection logic to Stats/Usage collectors ([Telemetry] Move all collection logic to Stats/Usage collectors #89709)
• Only allow admins (and Read-Only Admins) to fetch unencrypted telemetry (Only allow admins (and Read-Only Admins) to fetch unencrypted telemetry #96538) / FTR test helpers using kibana_system ([Telemetry] Test helpers for kibana_system #92825)
• Move usage collection to core ([Usage collection] move plugin to core #101146)

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)