datasource/gitlab_project: don't fail if user has no access to push rules

This avoids errors like this: https://gitlab.com/gitlab-org/frontend/renovate-gitlab-bot/-/jobs/3784211549#L45

docs/data-sources/project.md

@@ -70,7 +70,7 @@ data "gitlab_project" "example" {
 - `path` (String) The path of the repository.
 - `pipelines_enabled` (Boolean) Enable pipelines for the project.
 - `printing_merge_request_link_enabled` (Boolean) Show link to create/view merge request when pushing from the command line
-- `push_rules` (List of Object) Push rules for the project. (see [below for nested schema](#nestedatt--push_rules))
+- `push_rules` (List of Object) Push rules for the project. Push rules are only available on Enterprise plans and if the authenticated has permissions to read them. (see [below for nested schema](#nestedatt--push_rules))
 - `remove_source_branch_after_merge` (Boolean) Enable `Delete source branch` option by default for all new merge requests
 - `repository_access_level` (String) Set the repository access level. Valid values are `disabled`, `private`, `enabled`.
 - `repository_storage` (String) Which storage shard the repository is on. (administrator only)