Make blob_permalink_embed FF derisk; add roll-out issue

What does this MR do and why?

With the merge of Add blob snippet embed support to content editor (!246840 - merged) and Blob embeds respect show_diff_preview_in_email (!251058 - merged), the feature (Render blob line of code in GFM (#300855 - closed)) no longer has any blockers for changing to a derisk FF. Make the change and add the roll-out issue link.

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