Bring back the Slack notifications docs

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What does this MR do?

I noticed this was removed in !154008 (merged). The decision was partially correct due to the removal dates added in 6922651b. At that time, the Slack notifications would be removed in 17.0, but in 73187d9f it's decided to further expand the removal to 18.0.

Unfortunately, the removal date wasn't updated as well, so when the clearing process began, the page was moved.

This MR brings the page back and updates the removal date to that of GitLab 18.0.

PS. I only realized this when I saw the Slack notification in #docs-changes about the nav item, and because I worked on gitlab-docs#1787 (closed), so I knew it was being removed in 18.0 😄 Pretty lucky.

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