ci: Bump Taskgraph to 7.0.0

[ahal]

Description

This bumps Taskgraph to version 7 which contains several backwards incompatible changes. There's no urgency to land this, other than I'm going through and updating repos for consistency and while the backwards incompatible changes are fresh in my mind.

Checklist

• My code follows the style guidelines for this project
• I have not added any packages that contain high risk or unknown licenses (GPL, LGPL, MPL, etc. consult with DevOps if in question)
• I have performed a self review of my own code
• I have commented my code PARTICULARLY in hard to understand areas
• I have added thorough tests where needed

[ahal]

Note that because this renames taskcluster/ci -> taskcluster/kinds, there's a high risk this will cause conflicts with other pending work (though resolving should be as easy as a git mv).