fix scalatra on Windows (#16395)

Author: martin-mfg

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/ScalatraServerCodegen.java

@@ -200,6 +200,6 @@ public OperationsMap postProcessOperationsWithModels(OperationsMap objs, List<Mo
     }
 
     private String sourceFolderByPackage(String packageName) {
-        return sourceFolder + File.separator + packageName.replaceAll("[.]", File.separator);
+        return sourceFolder + File.separator + packageName.replace('.', File.separatorChar);
     }
 }