[Bug] serverjwtauth sample doesn't work with newer servers

[cretz]

Describe the bug

Multiple server changes were made since this sample was written. Specifically we need to:

• Update README to have them set USE_INTERNAL_FRONTEND=true env var and SERVICES=frontend:internal-frontend:history:matching:worker
• Update https://github.com/temporalio/samples-go/blob/6b93fe45a8824a56ac70f30832d262b94921a80d/serverjwtauth/key/main.go#L95C35-L95C35 to change system:admin to temporal-system:admin