Add event listener to check that the kernel.secret parameter isn't set to the default

After reading https://github.com/symfony/symfony/pull/6463, I think it's becoming increasingly important that `kernel.secret` is *absolutely* set to something secure.

I have seen (and also been guilty of for quick builds) projects that don't bother changing the secret. This PR adds a warning to the logger when this is detected.

Perhaps a compiler pass could be used to unsubscribe from the event once the issue has been resolved.

Note: threw this together in a few mins... might not comply with conventions