[META] In the next commit, we move the clearing of pindexBestHeader to ChainstateManager::Unload()
This is a partial backport of [[https://github.com/bitcoin/bitcoin/pull/24909 | core#24909]]
https://github.com/bitcoin/bitcoin/pull/24909/commits/0d567daf23c9fcb2d95b38913ee45a8b0ba3b027
Depends on D13043