Simplify the steps to make changes to config.json (#5186)

This change introduces following simplified steps to follow 
during config migration.

```
 // Steps to move from version N to version N+1
 // 1. Add new struct serverConfigVN+1 in config-versions.go
 // 2. Set configCurrentVersion to "N+1"
 // 3. Set serverConfigCurrent to serverConfigVN+1
 // 4. Add new migration function (ex. func migrateVNToVN+1()) in config-migrate.go
 // 5. Call migrateVNToVN+1() from migrateConfig() in config-migrate.go
 // 6. Make changes in config-current_test.go for any test change
```

cmd/gateway-startup-msg.go

@@ -45,7 +45,7 @@ func printGatewayStartupMessage(apiEndPoints []string, backendType string) {
 // Prints common server startup message. Prints credential, region and browser access.
 func printGatewayCommonMsg(apiEndpoints []string) {
 	// Get saved credentials.
-	cred := serverConfig.GetCredential()
+	cred := globalServerConfig.GetCredential()
 
 	apiEndpointStr := strings.Join(apiEndpoints, "  ")
 	// Colorize the message and print.