[CI] Migrate configuration file to YAML

Summary:
YAML is more appropriated for a configuration file, and has the key
advantage of supporting comments.

There is no change in behavior.

Test Plan: Run any CI build.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7357