Fixing docker-compose gotenberg image version to 6

[crazychatting]

The gotenberg docker-image was updated with their current version 7.x.
We need to use version 6 for the current API usage.

Otherwise the gotenberg conversion fill fail fith the following:
[2022-01-16 00:18:50,040] [ERROR] [paperless.consumer] Error while consuming document document.docx: Error while converting document to PDF: 404 Client Error: Not Found for url: http://gotenberg:3000/convert/office

see https://gotenberg.dev/docs/6.x/introduction

[crazychatting]

dammit! Obiously this should be a pull request to the dev-branch... should I create a new pull request?

[crazychatting]

closing this pull request due to duplicate. It seems like the dev-branch already addressed this bug.
see

• fix(gotenberg): use command flags #1438
• [BUG] Error while converting document to PDF: 404 Client Error: Not Found for url: http://gotenberg:3000/convert/office #1250
• Problem with adding documents other than PDF: Not Found for url: http://gotenberg:3000/convert/office #1455
• fix(tika): adapt to Gotenberg 7 API #1262

so changing the image to version 6 should only be a quickfix until there is a new release.