bug symfony#38422 [SecurityGuard] Implement PostAuthenticationGuardToken::getFirewallName() (derrabus)

This PR was submitted for the master branch but it was merged into the 5.x branch instead.

Discussion
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[SecurityGuard] Implement PostAuthenticationGuardToken::getFirewallName()

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix symfony#38421
| License       | MIT
| Doc PR        | N/A

Commits
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66fbf0a [SecurityGuard] Implement PostAuthenticationGuardToken::getFirewallName().

Author: nicolas-grekas

src/Symfony/Component/Security/Guard/Token/PostAuthenticationGuardToken.php

@@ -69,6 +69,11 @@ public function getProviderKey()
         return $this->providerKey;
     }
 
+    public function getFirewallName(): string
+    {
+        return $this->getProviderKey();
+    }
+
     /**
      * {@inheritdoc}
      */