[linters] Bump required black version in CONTRIBUTING.md
Summary: linter throws an error if you arc diff with 23 now that we are requiring 24
Test Plan: Confirm documentation matches version required by D15374
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D15387