upstream CI: Pin ansible-lint version to 6.20 series

The release version 6.21.0 of ansible-lint introduced a bug that breaks
the reporting of 'warning' messages. [1]

This patch pins ansible-lint version to the latest one in the 6.20
series, so that it can still be used to check pull requests.

[1]: https://github.com/ansible/ansible-lint/issues/3853