danger-review: Show deprecation warning also in `script:`

What does this MR do?

This MR moves the deprecation warning for danger-review from before_script to script because sometimes before_script is overridden and thus the warning never shown.

How to verify on CI?

See https://gitlab.com/gitlab-org/quality/pipeline-common/-/jobs/6847386686#L108

Check-list

• Ensure commits include Changelog: trailer
• Verify the change in all affected pipelines.

Refs

• gitlab-org/components/danger-review!4 (merged)
• gitlab-org/gitlab!152943 (merged)