Refactor manual variables form

What does this MR do and why?

This refactors the manual variables form for triggering a manual job.

• Use's one set of inputs instead of two sets (no need)
• Removes technical debt with legacy classes and moves to design system components
• Improves the unit test coverage
• Adds feature test to ensure variable values are passed correctly from FE to BE

Screenshots or screen recordings

before	after

Manual testing to ensure variable values are passed correctly

How to set up and validate locally

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

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #326809 (closed)