Enable use_merge_base_for_security_widget by default

What does this MR do and why?

Describe in detail what your merge request does and why.

Rollout issue: #407762 (closed)

Implementation issue: #295167 (closed)

Epic: &10092 (closed)

Enable use_merge_base_for_security_widget by default. This flag has been globally enabled in production since 2023-04-26 at 20:05 UTC. Since we haven't identified any regressions or performance degradations as a result of this change, we're ready to enable the flag by default.

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