bug #41367 [Ldap] Avoid calling the deprecated getUsername() (derrabus)

This PR was merged into the 5.3 branch.

Discussion
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[Ldap] Avoid calling the deprecated getUsername()

| Q             | A
| ------------- | ---
| Branch?       | 5.3
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | N/A
| License       | MIT
| Doc PR        | N/A

Commits
-------

080f81f1f4 [Ldap] Avoid calling the deprecated getUsername()

Author: chalasr

Security/LdapUser.php

@@ -121,7 +121,7 @@ public function isEqualTo(UserInterface $user): bool
             return false;
         }
 
-        if ($this->getUsername() !== $user->getUsername()) {
+        if ($this->getUserIdentifier() !== $user->getUserIdentifier()) {
             return false;
         }