Fix R004: ResourceData.Set() incompatible value type
Check: R004
./gitlab/data_source_gitlab_user.go:198:22: R004: ResourceData.Set() incompatible value type: *time.Time
./gitlab/resource_gitlab_group_membership.go:154:22: R004: ResourceData.Set() incompatible value type: *github.com/xanzy/go-gitlab.ISOTime
./gitlab/resource_gitlab_service_slack.go:148:40: R004: ResourceData.Set() incompatible value type: func(b []byte) error
./gitlab/resource_gitlab_service_slack.go:149:38: R004: ResourceData.Set() incompatible value type: func(b []byte) error
We should fix these reported issues, and then remove this check from the list of ignored checks in the gitlabproviderlint make target.
To reproduce, run go run ./gitlabproviderlint ./gitlab
(pending #432)