Update tech_stack.yml - removing Hello Sign (no longer used; replaced with DocuSign)

Tech Stack Update

Please do not merge this MR before the Business Systems Analysts have reviewed and approved.

Please take into account:

• If changing provisioners: You must also open an Update Tech Stack provisioner issue so all templates and Tech Stack information can be updated.
• If completing a system offboarding:
  • Follow the steps in the Tech Stack offboarding section

/cc @lisvinueza