Ignore token not having the getCredentials() method during decode

[fmarchalemisys]

The getCredentials() method was removed from TokenInterface in Symfony 6. Beside, some tokens, such as TestBrowserToken, return null instead of the expected string type. See issue #1040.

Decoding a jwt token is only meaningful when the token is a JWTPostAuthenticationToken. Any other token can be ignored.

To fix the deprecation, the user's code must do something like this:

if (!$token instanceof JWTPostAuthenticationToken) {
    return;
}
try {
    $jwtPayload = $this->jwtManager->decode($token);
} catch (JWTDecodeFailureException) {
    return;
}
if ($jwtPayload === false) {
    return;
}
// process $jwtPayload…    

Now, the questions:

1. Do we deprecate the call with anything other than a JWTPostAuthenticationToken with the aim of restricting the type to public function decode(JWTPostAuthenticationToken $token): array|bool later on?
2. Do we simply ignore tokens other than JWTPostAuthenticationToken without deprecation to make the use of the decode method easier?

This PR is currently aiming at the first solution but solution 2 simply requires to remove the trigger_deprecation call.