[flaky-tests] AdminApiSchemaTest#testSchemaInfoApi (apache#14508)

* [flaky-tests] AdminApiSchemaTest#testSchemaInfoApi

* use custom json schema

* remove old comment

### Motivation
This is the same fix applied here apache#12461. The problem with the other pull is that the `AbstractSchema#clone()` method does return the same instance, so the fix is not useful at all.

I see this test also failing in 2.8 and 2.9 branch, I recommend to cherry-pick it.

### Modifications
* Create a new INT schema for the test purpose

- [x] `no-need-doc`

(cherry picked from commit 32c3cd1)

pulsar-broker/src/test/java/org/apache/pulsar/schema/SchemaTest.java

@@ -703,8 +703,7 @@ public void testNullKeyValueProperty() throws PulsarAdminException, PulsarClient
         map.put("key", null);
         map.put(null, "value"); // null key is not allowed for JSON, it's only for test here
 
-        // leave INT32 instance unchanged
-        final Schema<Integer> integerSchema = Schema.INT32.clone();
+        final Schema<Integer> integerSchema = Schema.JSON(Integer.class);
         ((SchemaInfoImpl) integerSchema.getSchemaInfo()).setProperties(map);
 
         final Consumer<Integer> consumer = pulsarClient.newConsumer(integerSchema).topic(topic)