Remove deprecated attributes from various resources

This merge request removes deprecated configuration options from GitLab Terraform provider documentation. The changes clean up outdated settings that were scheduled for removal in version 19.0, such as old branch protection settings, legacy integration options, and deprecated project configuration fields. Users are guided to use newer, replacement settings instead. Additionally, one new access level option is added for group sharing. This is essentially housekeeping work to modernize the documentation by removing obsolete features and directing users toward current best practices.

Part of #6512