GameUpdater's internal Subject is not disposed when the server is destroyed.

Expected behavior:
- When the server starter is destroyed, it should clean up the server update thread.
- It should clean up the game update thread.
- It should cancel auto-save.
- It should also dispose the GameUpdater update stream so no leftover Subject remains alive.

--- Original GitHub issue (auto-fetched) ---
Title: GameUpdaterの中身のSubjectをDisposeしていない

Body:
ごめん!!!!
