📣 Discuss required upgrade stops in the "Upgrade GitLab" docs

As a part of the Upgrade improvements working group, we determined that upgrade stops need to be discussed on specific upgrade-related docs pages. The specific pages to update are:

• Create a GitLab upgrade plan
  • This page no longer needs to be updated as !169224 (merged) was merged after I created this issue.
• Upgrade GitLab

We do have information about required upgrade stops on the upgrade paths page and in the Upgrade Path tool. However, we do not mention the existence of required upgrade stops on the pages linked above. The proposed change will make it easier for customers to be aware of upgrade stops in general and aware of whether their planned upgrade will involve a required stop sooner in the upgrade process.

There is some related work on highlighting upgrade stops in gitlab-org#12542.

Thanks to @.luke, @bprescott\_, @dorrino and @mkozono for their contributions to this conversation.