refactor: Improve use of explicit keyword

Summary:
Notes:

• the linter part of the code is not applicable. We got rid of that linter in D11063 as it was obviously not very good at its job.
• most of the work was already done in D6549
• some of these constructors have two parameters in Bitcoin ABC

This is a backport of core#20530
https://github.com/bitcoin/bitcoin/pull/20530/commits/1e62350ca20898189904a88dfef9ea11ddcd8626

Test Plan:
with clang and gcc
ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11557