bump minimum required boost version to 1.64

Summary:
This is the same version as the current minimum supported by Bitcoin Core.

The packaged versions of boost on the oldest supported distros are:

• Ubuntu 20.04: 1.71
• Debian 10: 1.67
• CentOS 8: 1.68

Depends on D13985

Test Plan: cmake .. -GNinja && ninja all check-all to the test change to CMakeLists.txt

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13986