Enable runners_finder_all_available FF by default

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR enables by default [Feature flag] Rollout of `runners_finder_all_a... (#374525 - closed), introduced in %15.5 by GraphQL: Add ALL_AVAILABLE membership value (!96770 - merged). The intent of this FF is to allow disabling an expensive query if needed.

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