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

CHANGES.rst

@@ -3,6 +3,8 @@ Version 3.1.1
 
 Unreleased
 
+-   Fix signing key selection order when key rotation is enabled via
+    ``SECRET_KEY_FALLBACKS``. :ghsa:`4grg-w6v8-c28g`
 -   Fix type hint for `cli_runner.invoke`. :issue:`5645`
 -   ``flask --help`` loads the app and plugins first to make sure all commands
     are shown. :issue:5673`