Deepl deprecates GET-Request and sending the api-key in the body

[usox]

The deprecation of GET-Request wont affect us, but the documentation shows no sign of the support for form-encoded data. So we better switch to json here.
Also we need to send the api-key via header instead sending it as body-param.

https://developers.deepl.com/docs/resources/breaking-changes-change-notices/march-2025-deprecating-get-requests-to-translate-and-authenticating-with-auth_key