Hi!

Symfony version(s) affected: 5.1.1

Description
Maybe BC, maybe just a docs issue:
I have remember me functionality on my site, as shown in the docs, here: https://symfony.com/doc/current/security/remember_me.html

After 5.1.1 I got the error in the title.

How to reproduce

Possible Solution
readd the variable or update the docs to use secret: '%env(APP_SECRET)%' instead of secret: '%kernel.secret%'

Additional context
Maybe related to this PR: #37154