From 0.14 (2017 Mar) until before 0.19 (2019 Nov), the height of the last
block pruned was returned, subject to a bug if there were blocks left unpruned
due to sharing files with later blocks.
In #15991, this was "fixed" to the current implementation, introducing a new
bug: now, it returns the first *unpruned* block.
Since the user provides the parameter as a block to include in pruning, it
makes more sense to fix the behaviour to match the documentation.
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/24629 | core#24629]]