[TASK] Remove upper version constraint for deeplcom/deepl-php

Due to the outdated `DeepL Mock Api Server` version testing
the extension with a newer `deeplcom/deepl-php` client badly
failed and has been mitigated by adding a composer version
constraint to avoid breaking dependency versions.

The container image for the `DeepL Mock Api Server` has been
exchanged with a up-to-date once which rebuilds and releases
itself automatically.

Used command(s):

```bash
composer require --no-update \
  "deeplcom/deepl-php":"^1.6.0"
```

composer.json

@@ -70,7 +70,7 @@
 		"ext-curl": "*",
 		"ext-json": "*",
 		"ext-pdo": "*",
-		"deeplcom/deepl-php": ">=1.6.0 <=1.8.0",
+		"deeplcom/deepl-php": "^1.6.0",
 		"typo3/cms-backend": "^11.5 || ^12.4",
 		"typo3/cms-core": "^11.5 || ^12.4",
 		"typo3/cms-extbase": "^11.5 || ^12.4",