Update generated URLs to point to v3

This issue has been migrated from [#11474](https://github.com/matrix-org/synapse/issues/11474).

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Related to #11079, we generate some URLs internally that point to `r0` URLs, some places we do this is for:

* Callback URLs used in SSO
* Attempting to fetch media from remote servers

Maybe other places. We should eventually update these to use `v3` URLs. The ones that generate URLs that go back to ourselves should be easy enough (if people have their reverse proxies configured properly), but the ones to other servers are a bit more complicated. For federation we try `v2` URLs then fallback to `v1` URLs, but that's rather annoying. Not sure if there's something better to do.