chore(config): upgrade to gosettings v0.4.0

- drop qdm12/govalid dependency
- upgrade qdm12/ss-server to v0.6.0
- do not unset sensitive config settings (makes no sense to me)

internal/configuration/settings/errors.go

@@ -3,6 +3,7 @@ package settings
 import "errors"
 
 var (
+	ErrValueUnknown                    = errors.New("value is unknown")
 	ErrCityNotValid                    = errors.New("the city specified is not valid")
 	ErrControlServerPrivilegedPort     = errors.New("cannot use privileged port without running as root")
 	ErrCategoryNotValid                = errors.New("the category specified is not valid")