Roll out blob permalink embeds

What does this MR do and why?

Rolls out blob permalink embeds, under the blob_permalink_embed FF!

See Render blob line of code in GFM (#300855 - closed), and the roll-out issue, [FF] `blob_permalink_embed` -- embed snippets o... (#623019 - closed).

We have a dashboard showing uptake of the feature.

Screenshots or screen recordings

This is already enabled prod-wide, so you can test it right now per the documentation. For example:

https://gitlab.com/gitlab-org/gitlab/-/blob/41292ede4f62c0c977355d211d3bbf598991e0d7/lib/banzai/filter/repository_blob_embed_filter.rb#L113-123

Renders like this:

lib/banzai/filter/repository_blob_embed_filter.rb Lines 113 to 123

113 114 115 116 117 118 119 120 121 122 123

      def render_embed(embed)
        html = Gitlab::BlobEmbed::Renderer.new(
          project: embed[:target],
          sha: embed[:sha],
          path: embed[:path],
          from: embed[:from],
          to: embed[:to],
          blob: embed[:blob],
          cross_project: cross_project?(embed[:target]),
          for_email: for_email?
        ).render

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