Update alerts

What does this MR do and why?

Minor adjustments to Alert boxes:

• Updates disclaimer text to match with a recent MR on the existing site (gitlab-org/gitlab-docs!4735 (merged))
• Fixes background color for Info alert

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.