validation: Don't error if maxsigcachesize exceeds uint32::max

Summary:
Instead clamp it to uint32::max if it exceeds it.

Co-authored-by: Anthony Towns <aj@erisian.com.au>

This is a partial backport of core#30141
https://github.com/bitcoin/bitcoin/pull/30141/commits/ab14d1d6a4a8ef5fe5013150e6c5ebcb5f5e4ea9

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18855