Symfony version(s) affected: 5.2.0-beta1

Description

I've upgraded a Symfony 5.1 application that uses a Guard authenticator to Symfony 5.2-beta1. The following deprecation is logged:

User Deprecated: Since symfony/security-http 5.2: Method "Symfony\Component\Security\Guard\Token\PostAuthenticationGuardToken::getProviderKey()" has been deprecated, rename it to "getFirewallName()" instead.

I believe my application can't do much about it because the class in question is a Symfony class.

How to reproduce
Use a guard authenticator.

Possible Solution
Implement getFirewallName() on PostAuthenticationGuardToken.

Additional context
The deprecation was introduced with #37942, but apparently the Guard component has not been updated accordingly. /cc @wouterj