Set detailsURL in update XML to be useful for 13.5 users

As mentioned in #43168 (closed), users on 13.5 are expected to get an update with an "unsupported" attribute. See our tor-browser patch.

When this happens, there is existing logic from mozilla to show "unsupported" messages in a doorhanger, about:preferences and the about dialog. These all have "Learn more" links.

As far as I can tell, these "Learn more" links will point to the detailsURL of the corresponding updates XML.

We should make sure that detailsURL works for 13.5 users, at least for a while. I'm not sure how this works exactly:

1. If we can set detailsURL independently for 13.5 users, then we can use the same url as our banners. Move this issue to tor-browser-build, if so. Blocked on #44667 (closed).
2. If this detailsURL comes from the 15.0 updates, then it will probably point to our blog posts for the 15.0 releases. In which case, we likely want something in the blog post to help users that end up there.
3. Or we can patch the 13.5 update handler code to overwrite detailsURL when we set "unsupported".

@morgan @pierov