Fix caching large snippet HTML content on MySQL databases

What does this MR do?

Increases the size limit of the snippets.content_html column on MySQL databases

Are there points in the code the reviewer needs to double check?

Other _html columns? This is the only column with an uncached counterpart that has had its MySQL limit increased, so I think this is a complete fix.

This migration will run O(N) with the number of snippet rows. MySQL users are getting a raw deal here, but it doesn't seem that there's a good option: https://serverfault.com/questions/6771/altering-a-column-length-on-a-large-innodb-table-with-minimal-downtime

Why was this MR needed?

This bug breaks caching large snippets on MySQL systems.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #31647 (closed)