Revise deprecation to use warning box
Completes a post-merge review (thanks, @.luke) of the work merged on an emergency basis in !57434 (merged). It shifts the deprecation declaration from the H1, to the warning box as the style guide describes: https://docs.gitlab.com/ee/development/documentation/styleguide/index.html#warning
Small tone / style revisions to the contents of the box as well.
- Related to #27954 (closed)
- Related to #325851 (closed)
- Related to gitlab-com/gl-infra/production#4054 (closed)
- Closes #325924 (closed)