Require at least Git v2.28.0

The Git version check hasn't been updated in a while and is still at
v2.22.0, even though the minimum required version of Git is in fact
v2.25.0. Now that Gitaly has bumped its requirements to v2.28.0, let's
fix the system check to match.

lib/system_check/app/git_version_check.rb

@@ -7,7 +7,7 @@ module SystemCheck
       set_check_pass -> { "yes (#{self.current_version})" }
 
       def self.required_version
-        @required_version ||= Gitlab::VersionInfo.parse('2.22.0')
+        @required_version ||= Gitlab::VersionInfo.parse('2.28.0')
       end
 
       def self.current_version