[META] This commit should be followed up by removing the comments and assertions meant only to show that the change is correct.
GetSpendHeight only acts on BlockManager.
This is a backport of core#20749 [6/17]
https://github.com/bitcoin/bitcoin/pull/20749/commits/e4b95eefbc700ebc915bec312f77477ce3e87a7e
Depends on D11174