[linter] Bump clang-format to version 12

Summary: We use clang-12 and clang-tidy-12, and already install version 12 on CI. Update the documentation accordingly.

Test Plan:

arc lint --everything

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12185