Add ci_forward_deployment_rollback_allowed to gitlab_project

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Summary

Generated by GitLab Duo

This change adds support for the existing GitLab CI/CD feature called "ci_forward_deployment_rollback_allowed" to the Terraform GitLab provider. This setting allows job retries even when a deployment job is outdated.

The implementation adds this new boolean field to both the project resource and projects data source, including proper documentation, schema definitions, and handling in the code that maps between Terraform state and GitLab API objects.

The change also includes test updates to verify the new functionality works correctly. This feature appears to be related to the existing "ci_forward_deployment_enabled" setting and provides more flexibility in GitLab's deployment pipeline behavior.

Related issue: #6527 (closed)