MM-46911: Skip flaky TestGetTopDMsForUserSince (#20962)

https://mattermost.atlassian.net/browse/MM-46911

```release-note
NONE
```

api4/insights_test.go

@@ -897,6 +897,7 @@ func TestGetTopInactiveChannelsForTeamSince(t *testing.T) {
 }
 
 func TestGetTopDMsForUserSince(t *testing.T) {
+	t.Skip("MM-46911")
 	th := Setup(t).InitBasic()
 	defer th.TearDown()