Simplify Packer templates for Windows and Ubuntu images

[feliasson]

Description

Reference: #12305

I like this refactor but can we please stop introducing new ways of referencing the same image to generate.
The goal here is consistency, and reusing the same approach for referencing images helps maintain that across the codebase.

In this I am simply reusing ImageType as input for the image_os variable, which is used in the new map in locals, instead of adding extra code and creating a new reference for the same image type.

Current reference examples for image

Windows2025
windows-2025
win25

Ubuntu2404
ubuntu-24_04
ubuntu24

I’ll leave this PR as an example of an approach that avoids introducing additional references and aims to stay more consistent. Please consider.

Related issue:

Check list

• Related issue / work item is attached
• Tests are written (if applicable)
• Documentation is updated (if applicable)
• Changes are tested and related VM images are successfully generated