change workspace resolver to "2"

[hasezoey]

This PR changes the resolver for the workspace to be version 2, because all lower crates use edition 2021 which default to resolver 2, as per cargo warning:

warning: some crates are on edition 2021 which defaults to `resolver = "2"`, but virtual workspaces default to `resolver = "1"`
note: to keep the current resolver, specify `workspace.resolver = "1"` in the workspace root's manifest
note: to use the edition 2021 resolver, specify `workspace.resolver = "2"` in the workspace root's manifest

[gyscos]

Thanks for the work!