refactor: modernize CScriptBase definition

Summary:
This is a partial backport of core#32279
https://github.com/bitcoin/bitcoin/pull/32279/commits/65ac7f6d4d1e26b7c065ed3dc0a17db822b08d71

Depends on D18813

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18814