Disable sandboxed_mermaid feature flag by default

What does this MR do and why?

Reverts !78283 (merged)

Enabling sandboxed_mermaid feature flag caused a regression issue with self-hosted instances.

The reason for the issue is that we have X-FRAME-OPTIONS set as DENY by default, which would not allow meramid iframe (from the same origin) to be rendered. !79043 (diffs) should fix that but this MR disables the feature flag in the meantime.

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