secret key rotation: fix key list ordering

The `itsdangerous` serializer interface[1] expects keys to be
provided with the oldest key at index zero and the active signing key
at the end of the list.

We document[2] that `SECRET_KEY_FALLBACKS` should be configured with
the most recent first (at index zero), so to achieve the expected
behaviour, those should be inserted in reverse-order at the head of
the list.

[1] - https://itsdangerous.palletsprojects.com/en/stable/serializer/#itsdangerous.serializer.Serializer

[2] - https://flask.palletsprojects.com/en/stable/config/#SECRET_KEY_FALLBACKS

src/flask/sessions.py

@@ -318,11 +318,12 @@ class SecureCookieSessionInterface(SessionInterface):
         if not app.secret_key:
             return None
 
-        keys: list[str | bytes] = [app.secret_key]
+        keys: list[str | bytes] = []
 
         if fallbacks := app.config["SECRET_KEY_FALLBACKS"]:
             keys.extend(fallbacks)
 
+        keys.append(app.secret_key)  # itsdangerous expects current key at top
         return URLSafeTimedSerializer(
             keys,  # type: ignore[arg-type]
             salt=self.salt,