Unify update pages

Can't find the similar issues but I know there are at least a few for improving the UX of this, so I wonder if they can be merged.

Replacing the "update manager" pages entirely via autoupdates and project browser would help. Then we'd be back to a single "Available updates" report. I'm not keen on spending any energy improving the UIs that should be deprecated and removed.

And yeah, there are a bunch of issues to improve the UI of just the available updates report itself...

Tempted to call this "outdated" at this point, although neither project browser nor autoupdates are actually ready to add to core, so it's a murky area.

With 📌 Deprecate/remove the ability to update a module from a URL and authorize.php Active and child issues complete, we're back to a single "Available updates" report, so closing this as "outdated".