[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 https://github.com/elastic/kibana/pull/91464#issuecomment-780018961

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:

• [x] [Application Usage] Better SO management (#87840)
• [x] Remove __LEGACY.appChanged (#91464)
• [x] Remove UI Metric's unused APIs (#91620)
• [x] Remove unused applicationUsageTracker (#92670)
• [x] Add telemetry plugins to the core eslint rules (#88816)
• [x] Remove use of any (#93183)
• [x] Use import type and export type where possible (#93184)
• [ ] Merge telemetry_management_section into the telemetry plugin (could be done as part of #56498)
• [ ] 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 (#89709)
• [x] Only allow admins (and Read-Only Admins) to fetch unencrypted telemetry (#96538) / FTR test helpers using kibana_system (#92825)
• [ ] Move usage collection to core (#101146)

[elasticmachine]

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