use java.io.Serializable (OpenAPITools#20815)

Author: martin-mfg

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaClientCodegenTest.java

@@ -3474,6 +3474,6 @@ public void testEnumWithImplements() {
         Map<String, File> files = new DefaultGenerator().opts(new ClientOptInput().openAPI(openAPI).config(codegen))
                 .generate().stream().collect(Collectors.toMap(File::getName, Function.identity()));
 
-        JavaFileAssert.assertThat(files.get("Type.java")).fileContains("Type implements org.openapitools.java.EnumInterface {");
+        JavaFileAssert.assertThat(files.get("Type.java")).fileContains("Type implements java.io.Serializable {");
     }
 }

modules/openapi-generator/src/test/java/org/openapitools/codegen/java/spring/SpringCodegenTest.java

@@ -5381,6 +5381,6 @@ public void testEnumWithImplements() {
         Map<String, File> files = new DefaultGenerator().opts(new ClientOptInput().openAPI(openAPI).config(codegen))
                 .generate().stream().collect(Collectors.toMap(File::getName, Function.identity()));
 
-        JavaFileAssert.assertThat(files.get("Type.java")).fileContains("Type implements org.openapitools.java.EnumInterface {");
+        JavaFileAssert.assertThat(files.get("Type.java")).fileContains("Type implements java.io.Serializable {");
     }
 }

modules/openapi-generator/src/test/resources/3_0/enum-implements.yaml

@@ -26,7 +26,7 @@ components:
           $ref: '#/components/schemas/Type'
     Type:
       type: string
-      x-implements: org.openapitools.java.EnumInterface
+      x-implements: java.io.Serializable
       enum:
         - Earth
         - Pegasi