Prepare autoscaling for release

[jasontedor]

This issue is a placeholder issue to track changes that are needed to prepare autoscaling for release.

• register the xpack.autoscaling.enabled setting in release builds (cf. Initial autoscaling commit #51161)
• include the autoscaling docs in release builds (cf. Exclude autoscaling docs from release docs #51190)
• remove enabling autoscaling in snapshot docs tests (it will be enabled by default) (cf. Enable autoscaling in snapshot docs tests #51474)
• remove ability to register autoscaling feature flag in release builds (cf. Permit autoscaling feature flag in release builds #52088)
• remove registering autoscaling feature flag in release builds (cf. Permit autoscaling feature flag in release builds #52088, Add autoscaling feature flag in release REST tests #52096)
  • docs (cf. Permit autoscaling feature flag in release builds #52088)
  • autoscaling REST tests (cf. Add autoscaling feature flag in release REST tests #52096)
  • autoscaling internal cluster tests (cf. db572af)
• document these steps for the development team

[elasticmachine]

Pinging @elastic/es-distributed (:Distributed/Autoscaling)