[Feature Request] [Tiered Caching] Remove feature flag for tiered caching

[peteralfonsi]

Is your feature request related to a problem? Please describe

Tiered caching will no longer be experimental in 3.0. We should remove its feature flag.

Describe the solution you'd like

After the feature flag is removed, users can always specify a store name via setting, and cache implementations including the default on-heap cache will always track their own stats as exposed in GET _nodes/stats/caches/....

Related component

Search:Performance

Describe alternatives you've considered

No response

Additional context

No response