Add support for lock_memberships_to_saml in Application Settings API

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Add missing attribute lock_memberships_to_saml to Application Settings API.

API docs: https://docs.gitlab.com/api/settings/

Implementation Guide

• Follow the CONTRIBUTING.md guide for setting up your local development environment.
• Clone the community fork of this project.
• Add LockMembershipsToSAML to the Settings and UpdateSettingsOptions structs in settings.go.
• Update relevant tests.