Simplify contributor setup by removing GOPATH dependency

Updates the contributor guide to reflect the shift to Go Modules
as the default build system from Go 1.13 onwards. Contributors
are no longer required to clone the repository within their GOPATH,
simplifying the setup process.

Author: yongruilin

contributors/guide/github-workflow.md

@@ -17,23 +17,14 @@ description: |
 
 ## 2. Clone fork to local storage
 
-Per Go's [workspace instructions][go-workspace], place Kubernetes' code on your
-`GOPATH` using the following cloning procedure.
+In your shell, define a local working directory as `working_dir`.
 
-[go-workspace]: https://golang.org/doc/code.html#Workspaces
-
-In your shell, define a local working directory as `working_dir`. If your `GOPATH` has multiple paths, pick
-just one and use it instead of `$GOPATH`. You must follow exactly this pattern,
-neither `$GOPATH/src/github.com/${your github profile name}/`
-nor any other pattern will work.
+[GO Modules]: https://go.dev/blog/using-go-modules
 
 ```sh
-export working_dir="$(go env GOPATH)/src/k8s.io"
+export working_dir="${HOME}/src/k8s.io" # Change to your preferred location for source code
 ```
 
-If you already do Go development on github, the `k8s.io` directory
-will be a sibling to your existing `github.com` directory.
-
 Set `user` to match your github profile name:
 
 ```sh