Add state mover for gitlab_project_integration_redmine

This code change adds support for automatically migrating users from an old GitLab integration resource to a new one. Specifically, it allows users to seamlessly move their Redmine integration configurations from the deprecated gitlab_integration_redmine resource to the newer gitlab_project_integration_redmine resource using Terraform's built-in migration features.

The implementation includes validation to ensure the migration only happens between the correct resource types and providers. It also includes comprehensive tests to verify the migration works properly, requiring Terraform version 1.8 or later since that's when cross-resource migration was introduced.

This change helps users upgrade their Terraform configurations without losing their existing integration settings or having to manually recreate them.

Part of #6717