Update the "Critical Security Release Process" page.
This makes a few refinements to the "Critical Security Release Process" page at https://about.gitlab.com/handbook/engineering/critical-release-process/ but leaves the page in place for now to avoid unexpected context-changing (forwarding to a different web site).
- The table of contents has been removed because there is only one section on the page.
- The layout has been changed from
handbook-page-toc
tomarkdown_page
. - The "Critical Security Release" level-1 heading has been replaced by a "Content Moved" level-2 heading.
- The content-moved text has been made friendlier and more descriptive.
cc @HSimpson