fix: prefill correct value for country in dummy data

src/Service/ParticipantDataPrefiller.php

@@ -142,7 +142,7 @@ class ParticipantDataPrefiller
         $address->street = 'Musterstr. 123';
         $address->postCode = '99999';
         $address->city = 'MusterOrt';
-        $address->country = 'Deutschland';
+        $address->country = 'DE';
         $participant->address = $address;
     }
 }

tests/Service/ParticipantDataPrefillerTest.php

@@ -276,6 +276,7 @@ class ParticipantDataPrefillerTest extends TestCase
         $this->assertSame('[email protected]', $participant->email);
         $this->assertSame('2006-01-15', $participant->dateOfBirth?->format('Y-m-d'));
         $this->assertSame('Musterstr. 123', $participant->address?->street);
+        $this->assertSame('DE', $participant->address?->country);
 
         CarbonImmutable::setTestNow();
     }