Change the default transaction isolation level to DEFAULT in AbstractJobExplorerFactoryBean

Resolves #4230

Author: fmbenhassine

spring-batch-core/src/main/java/org/springframework/batch/core/explore/support/AbstractJobExplorerFactoryBean.java

@@ -120,7 +120,7 @@ public void afterPropertiesSet() throws Exception {
 		if (this.transactionAttributeSource == null) {
 			Properties transactionAttributes = new Properties();
 			String transactionProperties = String.join(",", TRANSACTION_PROPAGATION_PREFIX + Propagation.SUPPORTS,
-					TRANSACTION_ISOLATION_LEVEL_PREFIX + Isolation.READ_COMMITTED);
+					TRANSACTION_ISOLATION_LEVEL_PREFIX + Isolation.DEFAULT);
 			transactionAttributes.setProperty("get*", transactionProperties);
 			transactionAttributes.setProperty("find*", transactionProperties);
 			this.transactionAttributeSource = new NameMatchTransactionAttributeSource();