Merge #14474: bitcoin-tx: Use constant for n pubkeys check

Summary:
1f01fe0257 bitcoin-tx: Use constant for n pubkeys check (Antoine Le Calvez)

Pull request description:

Use the constant for the maximum number of public keys in a multisig script defined in script/script.h instead of hardcoding it.

Tree-SHA512: 83e6c46df907944d0d993159955e402784415536d61fdb5a5becba2b042e37ad2a291b27301c1b169416cb71c823a571d82257512cd4a64848a27a24c875fcc6

Backport of Core PR14474
https://github.com/bitcoin/bitcoin/pull/14474/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4740