6.4 -> 6.5 cannot be cast to class

[fkowal]

Describe the bug
After upgrade from 6.4 -> 6.5
New implementation became unnecessarily type specific,
Making configuration in the users application uncenessarily incompatible during upgrade.

postProcess (AuthenticationProvider) became
post(OidcAuthorizationCodeAuthenticationProvider) causing a class cast exception

[fkowal]

MR: #17176