[Fleet] Move from `xpack.fleet.enableExperimental` config value to Feature Flags Service

[kpollich]

Kibana core introduced a generic Feature Flags Service in https://github.com/elastic/kibana/pull/188562. Today, Fleet implements its own feature flagging using a kibana.yml config value: xpack.fleet.enableExperimental. This value is allowlisted in Elastic Cloud, so users can opt their cloud deployments into feature flags for early testing.

We should move to the new, generic implementation provided by Kibana rather than continuing forward with our existing implementation.

• [ ] Be mindful of deprecations + breaking changes when making this change. Should we support both Fleet's experimental settings config and the new feature flags service at the same time for a smooth(er) migration?
• [ ] Ensure we can still opt into feature flags via the "user settings" UI in Elastic Cloud

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)