Change CAUTION to WARNING in alerts

I don't remember the exact reason for going with Caution instead of Warning, but I think we should change it to be consistent.

Steps

• Add WARNING alongside CAUTION in https://gitlab.com/gitlab-org/gitlab-docs/-/blob/master/lib/filters/admonition.rb.
• Change all occurrences of CAUTION into WARNING in the docs and update styleguide.
• Write a Vale rule to check for CAUTION and fail if found.
• Drop CAUTION from https://gitlab.com/gitlab-org/gitlab-docs/-/blob/master/lib/filters/admonition.rb