Update DDL migration script for IBM DB2 LUW

Resolves #5166

Author: fmbenhassine

spring-batch-core/src/main/resources/org/springframework/batch/core/migration/6.0/migration-db2.sql

@@ -1,2 +1,5 @@
--- IBM DB2 LUW (10.1+)
+-- The following is only applicable to Informix servers.
+-- IBM DB2 LUW (10.1+) does not support sequence renaming: https://www.ibm.com/docs/en/db2/12.1.x?topic=statements-rename
+-- To address this limitation, use a stored procedure or a Java-based program that retrieves
+-- the last sequence number and creates a new sequence starting from that value.
 RENAME SEQUENCE BATCH_JOB_SEQ TO BATCH_JOB_INSTANCE_SEQ;