rpc: Make pruneblockchain fetch old blocks if height is lower than pruned height #25026

Implements #24286.

This change allows pruned nodes to fetch older blocks from peers when pruning with a height that is lower than the current pruned height. This makes use of #20295. The peer used to fetch blocks is by default the peerid 0. I have added a functional test.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #19792 (rpc: Add dumpcoinstats by fjahr)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

Concept and Approach NACK. Feels too much like a hack to do it this way IMO. At the very least, I think it should require an explicit opt-in to the new behaviour, but I'm not sure there's a good use case either. It would make more sense to fetch old blocks as needed (eg, transparently in getblock)