Enable ruby_remote_branches_ls_remote flag by default

Robert Speicherrequested to merge
rs-ls-remote-by-default into master

This makes the experimental_remote_branches behavior the default unless explicitly disabled via feature flag.

The experimental behavior has been running for about three weeks in production without incident. This inversion of the flag's behavior is the final step to removing the flag entirely.

See #2670 (closed)

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