[cpp] move to c++20

Summary:
Bump the standard to c++20. There is some remaining compatibility code to be removed in follow-ups, but starting from this diff we can no longer compiler with the c++17 standard.

For now we don't bump the standard for the chronik code.

Depends on D18864

Test Plan:
ninja all check-all
guix builds

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18862