refactor: Use proper MAX_SCRIPT_ELEMENT_SIZE const
Summary:
This PR replaces well-known "magic" numbers with proper MAX_SCRIPT_ELEMENT_SIZE constants.
Backport of Bitcoin Core PR17439
Test Plan: ninja && ninja check
Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien
Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D7564