fix: use "No longer supported" as the default deprecationReason for d…

…eprecated fields with no reason specified

graphql/introspection/introspection.go

@@ -79,11 +79,17 @@ func (f *Field) DeprecationReason() *string {
 	}
 
 	reason := f.deprecation.Arguments.ForName("reason")
-	if reason == nil {
-		return nil
+
+	if f.IsDeprecated() {
+		if reason == nil {
+			defaultReason := "No longer supported"
+			return &defaultReason
+		} else {
+			return &reason.Value.Raw
+		}
 	}
 
-	return &reason.Value.Raw
+	return nil
 }
 
 func (f *InputValue) Description() *string {