Add upgrade guide for 19.0

This change adds a new upgrade guide document for version 19.0 of the Terraform GitLab Provider. The guide helps users migrate their existing configurations by explaining what needs to be updated due to breaking changes in GitLab 19.0.

The main changes users need to make include:

• Renaming many resources (like changing "gitlab_integration_jira" to "gitlab_project_integration_jira") and updating their Terraform state accordingly
• Replacing the old "gitlab_runner" with "gitlab_user_runner" which uses a newer authentication method
• Updating various attribute names and changing some boolean values (true/false) to text values (enabled/disabled)
• Moving project approval settings from a simple attribute to a dedicated resource for better management
• Removing support for deprecated cluster-related resources that GitLab no longer supports

The document provides step-by-step instructions and code examples to help users smoothly transition their infrastructure configurations to work with the new version.

Related to #6701 (closed)