Project settings missing an accessible REST API endpoint for some settings

<details> <summary> Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards. </summary> - [Label this issue](https://contributors.gitlab.com/manage-issue?action=label&projectId=278964&issueIid=584488) </details>  ### Summary As reported by [a Dedicated customer](https://gitlab.zendesk.com/agent/tickets/680217), there are currently two options in the GitLab project's setting UI that aren't made available via the REST API. This customer heavily relies on automation (including via terraform, which uses our REST API) to manage these options, so they would like a REST-compatible option made available. The settings are: * `protect_merge_request_pipelines` - Introduced in: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/188008 * `ci_display_pipeline_variables` - Introduced in: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/200232 * This does appear to have a GraphQL endpoint via `displayPipelineVariables`, but not the REST API.

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