Make `Feature.enabled?` to use `default_enabled: :yaml` by default

What does this MR do and why?

This makes Feature.enabled? to read feature flags by default from YAML, and raise exception for unknown feature flags.

This is first step towards getting rid of default_enabled: completely.

Related:

• #30228 (comment 760876708)
• Read this as well: !60923 (comment 598287437)

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