Add missing includes and forward declarations, remove unused ones
doc: Fix validation typo
Remove txindex migration code
PR description:
No supported version of Bitcoin used the legacy txindex, so all relevant nodes can be assumed to have upgraded. Thus, there is no need to keep this code any longer.
As a temporary courtesy, provide a one-time warning on how to free the disk space used by the legacy txindex.
This is a backport of core#22626
Depends on D12242