(fix): Pass API key in an HTTP header instead of request body

[Sed9yDataBank]

👋 today we noticed that translation bot was not working on Slack:

Failed to translate due to an unexpected response from translation server

I think I patched this last year Cellane/deep_thought#14 but in the wrong repository, and I just noticed now that this is the correct repo used for GHCR we're using for the deployed container we have:

ghcr.io/monstar-lab-oss/deepl-for-slack-elixir:master

00:37:21.909 [info] GET https://slack.com/api/conversations.replies -> 200 (313.530 ms)
00:37:22.002 request_id=GJn1fCXXXXXXMBWQ8h [info] GET /
00:37:22.002 request_id=GJn1fXXXXXXXrdMBWQ8h [info] Sent 200 in 275µs
00:37:22.979 [error] POST https://api.deepl.com/v2/translate -> 403 (1069.876 ms)

Docs

Thank you! 🫡