Related to #56205

Once all 1st party legacy plugins have been removed or migrated, we should disable legacy plugin discovery altogether, even for 3rd party plugins.

When we make this change, we also need to add a note to the "breaking changes" doc for 7.10

Blockers by team

@elastic/stack-monitoring-ui

• Migrate /api/settings route to Kibana platform #76416

@elastic/kibana-app-arch

• Replace usage of /elasticsearch/_msearch endpoint in Index Pattern Management UI #73993

@elastic/kibana-core-ui

• Replace usage of /elasticsearch/_msearch endpoint in tutorials #73992

@elastic/es-ui

• [Console] Legacy Config Deprecations in 7.10 #68497

@elastic/logs-metrics-ui

• [Logs App] Use new getActiveSpace function, abandon getInjectedVar() #68142

@elastic/kibana-gis

• Migrate legacy map UI settings #74562

@elastic/kibana-security

• remove legacy security plugin #75648 (blocked by Migrate status service and status page to New Platform #41983)
• Remove legacy spaces plugin #74532 (blocked by Migrate status service and status page to New Platform #41983)

@elastic/kibana-platform

• Migrate uiSettings defintions to NP #59755
• Beats Management plugin: migrate server-side code #70930
• Migrate ui/styles to New Platform #58369
• Migrate status service and status page to New Platform #41983
• Migrate CSP usage collector to New Platform #50511
• Migrate data folder creation to core #73002