Remove old integration resources 2

This merge request removes deprecated GitLab integration resources that were scheduled for removal in version 19.0. Specifically, it eliminates the old gitlab_integration_emails_on_push, gitlab_integration_redmine, and gitlab_integration_telegram resources along with their documentation and example files. These deprecated resources have been replaced by newer equivalents with gitlab_project_integration_ prefixes that provide the same functionality. The code cleanup also removes the backwards compatibility logic that was maintaining both old and new resource names, simplifying the codebase by keeping only the current resource implementations. This is a breaking change that requires users to migrate from the old resource names to the new ones before upgrading to version 19.0.

Part of #6512