Fix squash 'introduced in' version

doc/user/project/merge_requests/squash_and_merge.md

 # Squash and merge
 
-> [Introduced][ee-1024] in GitLab Enterprise Edition 8.16.
+> [Introduced][ee-1024] in GitLab Enterprise Edition 8.17.
 
 Squashing lets you tidy up the commit history of a branch when accepting a merge
 request. It applies all of the changes in the merge request as a single commit,