[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