[Provider] Add compatibility with Graylog 5.0

[AnonymousWP]

Graylog 5.0 has been released, see:

• https://www.graylog.org/post/graylog-5-0-a-new-day-for-it-secops/
• https://go2docs.graylog.org/5-0/changelogs/changelog.html#Graylog500

This likely means that some Terraform resources won't work anymore with Graylog 5.0, and thus need to be updated in order to make it compatible.

[AnonymousWP]

Can confirm issues do exist because of the API-upgrade: