Update prepare-supervisor-on-kind.sh to use vars for github secret

hack/prepare-supervisor-on-kind.sh

@@ -323,8 +323,8 @@ type: "secrets.pinniped.dev/github-client"
 metadata:
   name: my-github-provider-client-secret
 stringData:
-  clientID: REDACTED
-  clientSecret: REDACTED
+  clientID: "$PINNIPED_TEST_SUPERVISOR_UPSTREAM_GITHUB_CLIENT_ID"
+  clientSecret: "$PINNIPED_TEST_SUPERVISOR_UPSTREAM_GITHUB_CLIENT_SECRET"
 EOF
 fi
 
@@ -592,12 +592,6 @@ if [[ "$use_ad_upstream" == "yes" ]]; then
   echo
 fi
 
-if [[ "$use_github_upstream" == "yes" ]]; then
-  echo "    GitHub Username: your github username"
-  echo "    GitHub Password: your github password"
-  echo
-fi
-
 # Echo the commands that may be used to login and print the identity of the currently logged in user.
 # Once the CLI has cached your tokens, it will automatically refresh your short-lived credentials whenever
 # they expire, so you should not be prompted to log in again for the rest of the day.