refactor: Replace magic number with named constant

Summary:
This is a backport of core#18077 [2/13]
https://github.com/bitcoin/bitcoin/pull/18077/commits/28e2961fd6a2a9101fc08fb748430989291aaf7e

Depends on D12069

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12070