Resolve "Incorrect documentation for gitlab_group:project_creation_level"

This MR makes 4 changes to the gitlab_group resource, primarily documentation changes:

• It removes the "default" hints, which aren't always correct as noted in the associated issue, since self-hosted users can configure them differently
• It aligns the attribute descriptions of a couple attributes to the upstream API documentation (I performed a small sanity check just to re-align them while I was in making changes)
• It adds the Valid values are: blocks to attributes that accept a validated list of values, to align this resource to our style guide
• It updates log.Printf statements to instead use tflog, which will help with sanitizing tokens and just generally update to newer standards

Closes #6097 (closed)