refactor: more const annotations for uses of CBlockIndex*

Summary:
This is a partial backport of core#24515
https://github.com/bitcoin/bitcoin/pull/24515/commits/5be9ee3c54dcb396ff52fc8c8b7e4e6e39ec4a3b

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13020