Remove deprecated metrics embeds from GFM pipeline

What does this MR do and why?

Removes Prometheus & Grafana embeds from Gitlab-Flavored Markdown. These features were disabled behind a feature flag in %16.0, and we're now cleaning up all the files.

This MR removes the logic that identifies metrics embed links in markdown & appends a placeholder for the frontend to hook onto. This effectively nullifies the frontend code. The frontend will be cleaned up in another MR - !121538 (merged).

Note: This MR does not include helpers/dead code related to generating various embed attributes/urls/etc. I'll clean that stuff up in a another MR, since this MR is getting pretty big as-is. This MR contains only the GFM pipeline-related code.

How to set up and validate locally

Refer to !120310 (merged) for local testing instructions. Expected output is zero changes!

🎉 For each type of inline embed, we see no errors and the links still resolve (for clusters+grafana) or 404 (dashboard/alerts):

Note on 404s: In local dev, dashboards/alerts routes raise errors, but they'll 404 on production -> !122030 (comment 1415147196). That cleanup is in scope of #397138 (closed).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #397137 (closed)