Use FastAPIDeprecationWarning for openapi_prefix deprecation

The openapi_prefix parameter deprecation used logger.warning() which is
inconsistent with the rest of the codebase that uses warnings.warn() with
FastAPIDeprecationWarning. This makes the deprecation filterable with
standard Python warning filters and consistent with other deprecations.

fastapi/applications.py

@@ -1,3 +1,4 @@
+import warnings
 from collections.abc import Awaitable, Callable, Coroutine, Sequence
 from enum import Enum
 from typing import (
@@ -14,7 +15,11 @@ from fastapi.exception_handlers import (
     request_validation_exception_handler,
     websocket_request_validation_exception_handler,
 )
-from fastapi.exceptions import RequestValidationError, WebSocketRequestValidationError
+from fastapi.exceptions import (
+    FastAPIDeprecationWarning,
+    RequestValidationError,
+    WebSocketRequestValidationError,
+)
 from fastapi.logger import logger
 from fastapi.middleware.asyncexitstack import AsyncExitStackMiddleware
 from fastapi.openapi.docs import (
@@ -930,11 +935,13 @@ class FastAPI(Starlette):
             assert self.version, "A version must be provided for OpenAPI, e.g.: '2.1.0'"
         # TODO: remove when discarding the openapi_prefix parameter
         if openapi_prefix:
-            logger.warning(
+            warnings.warn(
                 '"openapi_prefix" has been deprecated in favor of "root_path", which '
                 "follows more closely the ASGI standard, is simpler, and more "
                 "automatic. Check the docs at "
-                "https://fastapi.tiangolo.com/advanced/sub-applications/"
+                "https://fastapi.tiangolo.com/advanced/sub-applications/",
+                category=FastAPIDeprecationWarning,
+                stacklevel=2,
             )
         self.webhooks: Annotated[
             routing.APIRouter,