Update 19.0 Migration guide for state move info

This update reorganizes and improves the documentation for upgrading to version 19.0 of the Terraform GitLab Provider. The changes make the upgrade guide much clearer and easier to follow by:

Better organization: The content is restructured with clearer headings and sections, separating different types of changes (renames, replacements, attribute changes) into distinct categories.

Improved formatting: Code examples are reformatted for better readability, with consistent indentation and clearer before/after comparisons.

Enhanced clarity: Instructions are rewritten to be more direct and actionable. For example, instead of saying "If you are using any of the following resources, you will need to complete these actions," it now clearly states "These resources are identical but have been renamed."

Added helpful details: New information is included, such as version requirements for using certain Terraform features and step-by-step migration processes.

Better visual cues: Warning callouts are added using the ~> notation to highlight important information like draft status and version requirements.

The actual upgrade instructions remain the same - this is purely about making the existing guidance more user-friendly and easier to understand for developers who need to upgrade their GitLab Provider configuration.

Related to #6717