Add state mover for gitlab_project_integration_telegram

This code change adds support for migrating from an old Telegram integration resource to a new one in a GitLab Terraform provider.

The main addition is a "state mover" function that automatically transfers configuration data when users upgrade from the deprecated gitlab_integration_telegram resource to the newer gitlab_project_integration_telegram resource. This migration feature validates that the move is coming from the correct source, copies all the settings (like tokens, room IDs, and notification preferences) to the new resource format, and ensures everything continues working seamlessly.

The code also includes comprehensive tests to verify this migration works correctly, requiring Terraform version 1.8 or later since that's when cross-resource migration was introduced. The test creates a Telegram integration using the old resource, then uses Terraform's "moved" block to transfer it to the new resource, and finally verifies everything still functions properly after the migration.

This allows users to upgrade their Terraform configurations without losing their existing GitLab Telegram integration settings or having to manually recreate them.

Part of #6717