pin to pnpm 6 for now

[pngwn]

Description

Please include:

• relevant motivation
• a summary of the change
• which issue is fixed.
• any additional dependencies that are required for this change.

Closes: # (issue)

Checklist:

• I have performed a self-review of my own code
• My code follows the style guidelines of this project
• I have commented my code in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[pngwn]

pnpm 7 was released and is installed by default breaking some stuff. I've pinned to 6 and going to merge so we can update other branches and get CI working again.