validation: Move GetSpendHeight to BlockManager

Summary:
[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

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11175