MM-24692: Add a since parameter to getGroups api (#14444)

* add a since parameter to getGroups api

* update for lint error

* when using since, return deleted groups as well.

* update flaky test, groups have same create time

Co-authored-by: mattermod <mattermod@users.noreply.github.com>

model/group.go

@@ -79,6 +79,7 @@ type GroupSearchOpts struct {
 	IncludeMemberCount     bool
 	FilterAllowReference   bool
 	PageOpts               *PageOpts
+	Since                  int64
 }
 
 type PageOpts struct {