Enable `sandboxed_mermaid` feature flag by default

FF Rollout issue - #349755 (closed)

What does this MR do and why?

Enable sandboxed_mermaid feature flag as it seems stable after the global rollout on production (around Jan 14th 0500 UTC).

The feature was released in !74414 (merged)

Note: There's a known issue with web-dev server throwing a stream of errors when rendering mermaid. However' the impact is negligible since it affects only GDK. We're trying to solve it here - !78196 (closed)

Screenshots or screen recordings

No visual changes

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