Document requirement of Git v2.28.0 for strong consistency

With release v13.5.0, Gitaly is migrating away from voting on
transactions via the pre-receive hook to the reference-transaction hook.
As this hook only exists in Git v2.28.0 and later, the use of strong
consistency thus mandates this version.

Adjust the documentation to spell out this new requirement.

doc/administration/gitaly/praefect.md

@@ -993,6 +993,8 @@ information, see the [strong consistency epic](https://gitlab.com/groups/gitlab-
 
 To enable strong consistency:
 
+- In GitLab 13.5, you must use Git v2.28.0 or higher on Gitaly nodes to enable
+  strong consistency.
 - In GitLab 13.4 and later, the strong consistency voting strategy has been
   improved. Instead of requiring all nodes to agree, only the primary and half
   of the secondaries need to agree. This strategy is enabled by default. To