undici override for @atproto/oauth-client-node #7

The @atproto/oauth-client-node package brings in undici v6, 7, and 8. Turns out 6 and 7 aren't compatible with workerd so I added an override to the pnpm-workspace.yaml to force them all to v8. Version 6 and 7 are there for older nodejs versions. Hopefully either a workerd-supported version comes out or they remove support for older versions.