Mark provider `token` as sensitive

Timo Furrerrequested to merge
token-sensitive into main

This change set mainly marks the token provider argument as sensitive - which is a breaking change for %15.7.

With that I've also added our first guide called Terraform GitLab Provider Version 15.7 Upgrade Guide (in a similar fashion the AWS provider does it, see here).

This change set also adds a test which ensures that we can upgrade from the last major provider version - from a "provider" interface-only pov. I'm not yet sure how useful it really is, but it helped me to ensure that the token change and the proto change can be upgraded seamlessly to I thought I'd add it.

Closes: #1357 (closed)

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