store previous release in `target_release` to avoid big scans

[davepacheco]

There are several places where we want to know both the current release and the previous one:

• in repo pruning (add background task to prune TUF repos #9107)
• in assembling a summary for the console ([nexus] Update status API #8897)
• in assembling the planning input

It's tempting to just select the last two rows of the target_release table, but that doesn't work because you can set the target release to itself after a MUPdate.

I'm adding a datastore function to do this in #9107 but it requires scanning back a bit, and in principle could have to either scan back really far or else give up. (I'm choosing to have it give up rather than scan unbounded.) But we could avoid all this if every time we set a new target release, we stored the previous distinct one.