chore: move to new witness repo

[mikhailswift]

We've split out the library code of witness into it's own repository,
separate from the CLI code. No other changes and this just switches
zarf to point to the new repository.

Description

Related Issue

Fixes # (issue)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist before merging

• Tests have been added/updated as necessary (add the needs-tests label)
• Documentation has been updated as necessary (add the needs-docs label)
• An ADR has been written as necessary (add the needs-adr label) [ 1 2 3 ]
• (Optional) Changes have been linted locally with golangci-lint. (NOTE: We haven't turned on lint checks in the pipeline yet so linting may be hard if it shows a lot of lint errors in places that weren't touched by changes. Thus, linting is optional right now.)

[jeff-mccoy]

Rolling this into #495, thanks.