resource/gitlab_project: deprecate `pipelines_enabled` and remove default
This change removes the explicit default for the now-deprecated pipelines_enabled
attribute.
This will allow to actually use the replacement field builds_access_level
.
However, as described in #1340 (comment 1181118519) there seems to be a bug in the upstream API. Thus, I won't contribute a test right now.
In general, I think we should remove the explicit defaults from this resource (or any resource actually). We can do this in a follow up MR (iterations)
Refs: #1340 (closed)