Use approve button workflow in danger CI

Use approve button workflow in danger CI

Since the LGTM label was deprecated in favor of using the Approve button
in gitlab, adjust the detection in danger bot.

Unfortunately, danger-python seems no longer maintained since 2020 and
MR approvals aren't available in its Python API (even though they're
supported in its Ruby/JS APIs). Going forward, let's use the more
comprehensive python-gitlab API.

It still makes sense to utilize the danger-python, since it handles the
integration with gitlab which doesn't need to be reimplemented as long
as it works - same with the other checks.