Update model use case request templates

What does this merge request do and why?

• Add some guidance on the use case and model request issue template
• Remove duplicate issue templates (issue templates must be placed under .gitlab/issue_templates).
• Rename issue templates to get them show up properly in issue dropdown.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• I've ran the affected pipeline(s) to validate that nothing is broken.
• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.