Allow save compat to exist for two versions.

We want to clean up eventually, but allowing it to exist in both develop
and the release branch makes cherry picks easier.

(cherry picked from commit c80e5b259f)

game/savecompat.py

@@ -41,7 +41,8 @@ def has_save_compat_for(
     """
 
     def decorator(func: Callable[..., ReturnT]) -> Callable[..., ReturnT]:
-        if major != MAJOR_VERSION:
+        # Allow current and previous version to ease cherry-picking.
+        if major not in {MAJOR_VERSION - 1, MAJOR_VERSION}:
             raise DeprecatedSaveCompatError(func.__name__)
         return func