Enable authenticated media by default

[turt2live]

Feel free to delay landing this PR as the team sees fit.

Pull Request Checklist

• Pull request is based on the develop branch
• Pull request includes a changelog file. The entry should:
  • Be a short description of your change which makes sense to users. "Fixed a bug that prevented receiving messages from other servers." instead of "Moved X method from EventStore to EventWorkerStore.".
  • Use markdown where necessary, mostly for code blocks.
  • End with either a period (.) or an exclamation mark (!).
  • Start with a capital letter.
  • Feel free to credit yourself, by adding a sentence "Contributed by @github_username." or "Contributed by [Your Name]." to the end of the entry.
• Code style is correct
  (run the linters)

[reivilibre]

when we land this, would be good to update the doc to say which version the default changed in

[anoadragon453]

We should shout about this change in the relevant release version's upgrade notes.

[rollecode]

I just realized this had broken all my images and only swithing it to false fixed things. I don't have knowledge enough to figure out why, but every single URL of my images showed {"errcode":"M_NOT_FOUND","error":"Not found"}. If this is going to be forced always on in the future, I guess I'll never upgrade again. If this is true, all image URLs cause 404.

Why this change and what it does? I'm using synapse-s3-storage-provider, is this taken into account?

[mwnu]

#18446