fix: resolve type errors in ObjectSerializer Date formatting

Author: 0x5457

modules/openapi-generator/src/main/resources/typescript/model/ObjectSerializer.mustache

@@ -182,11 +182,11 @@ export class ObjectSerializer {
             }
             if (format == "date") {
                 let month = data.getMonth()+1
-                month = month < 10 ? "0" + month.toString() : month.toString()
+                let monthStr = month < 10 ? "0" + month.toString() : month.toString()
                 let day = data.getDate();
-                day = day < 10 ? "0" + day.toString() : day.toString();
+                let dayStr = day < 10 ? "0" + day.toString() : day.toString();
 
-                return data.getFullYear() + "-" + month + "-" + day;
+                return data.getFullYear() + "-" + monthStr + "-" + dayStr;
             } else {
                 return data.toISOString();
             }