FileConfiguration uses executor for I/O (#5652)

* FileConfiguration uses executor for I/O

* Fix credentials tests

* Remove migrate_system_env as its very deprecated

supervisor/coresys.py

@@ -10,7 +10,7 @@ from functools import partial
 import logging
 import os
 from types import MappingProxyType
-from typing import TYPE_CHECKING, Any, TypeVar
+from typing import TYPE_CHECKING, Any, Self, TypeVar
 
 import aiohttp
 
@@ -102,6 +102,11 @@ class CoreSys:
         # Task factory attributes
         self._set_task_context: list[Callable[[Context], Context]] = []
 
+    async def load_config(self) -> Self:
+        """Load config in executor."""
+        await self.config.read_data()
+        return self
+
     @property
     def dev(self) -> bool:
         """Return True if we run dev mode."""