[chronik] Bump the build to c++20

Summary:
It requires to pin the CXX crate to a version that doesn't trigger the bug https://github.com/llvm/llvm-project/issues/67449 with the MacOS SDK.
Once the SDK is updated, we can remove the version pin.

Test Plan: Run the GUIX builds on all platforms.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D18868