Fix encode method with Symfony v6 (#69)

```
Declaration of FlixTech\AvroSerializer\Integrations\Symfony\Serializer\AvroSerDeEncoder::encode($data, $format, array $context = []) must be compatible with Symfony\Component\Serializer\Encoder\EncoderInterface::encode(mixed $data, string $format, array $context = []): string
```

integrations/Symfony/Serializer/AvroSerDeEncoder.php

@@ -51,7 +51,7 @@ public function supportsDecoding($format): bool
      * @throws \AvroSchemaParseException
      * @throws \FlixTech\SchemaRegistryApi\Exception\SchemaRegistryException
      */
-    public function encode($data, $format, array $context = [])
+    public function encode($data, $format, array $context = []): string
     {
         $this->validateEncodeContext($context);