update TransIP's Go client to v6

[skoef]

Description

TransIP API has switched to a REST-ful system (as opposed to the old SOAP API) some time ago and the available Go client was updated along with it, bumping it from v5 to v6. This patch replaces the old client with the new client.

Checklist

• Unit tests updated
• End user documentation updated

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[seanmalloy]

/kind cleanup

[njuettner]

/lgtm

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: njuettner, RobinGeuze, skoef

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