MM-53032: Fix module path after repo rename (#23689)

It was a good decision in hindsight to keep the public module as 0.x
because this would have been a breaking change again.

https://mattermost.atlassian.net/browse/MM-53032
```release-note
Changed the Go module path from github.com/mattermost/mattermost-server/server/v8 to github.com/mattermost/mattermost/server/v8.

For the public facing module, it's path is also changed from github.com/mattermost/mattermost-server/server/public to github.com/mattermost/mattermost/server/public
```

server/public/model/client4_test.go

@@ -28,7 +28,7 @@ func TestClient4TrimTrailingSlash(t *testing.T) {
 	}
 }
 
-// https://github.com/mattermost/mattermost-server/server/v8/channels/issues/8205
+// https://github.com/mattermost/mattermost/server/v8/channels/issues/8205
 func TestClient4CreatePost(t *testing.T) {
 	post := &Post{
 		Props: map[string]any{