feat: Add PackageRegistryAccessLevel to Project structs

Adds PackageRegistryAccessLevel field to the Project, CreateProjectOptions, and EditProjectOptions structs in projects.go.

GitLab's REST API already supports package_registry_access_level but the Go client did not expose it, making it impossible for Go developers to read or set this value. This also unblocks the Terraform provider from exposing package_registry_access_level in .tf files.

The new field uses the existing AccessControlValue type (disabled, private, enabled, public), following the same pattern as ContainerRegistryAccessLevel.

PackagesEnabled (old boolean field) is marked deprecated in all three structs in favor of the new granular access control field.

Closes #2255 (closed)