Fix non-nullable columns in MySQL migration for Spring Batch 4.3

[hpoettker]

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The database migration for MySQL to Spring Batch 4.3 currently misses NOT NULL constraints from BATCH_JOB_EXECUTION.CREATE_TIME and BATCH_STEP_EXECUTION.START_TIME.

If the current migration is applied to a Spring Batch database < 4.3, it effectively removes the existing NOT NULL constraints although they are still contained in schema-mysql.sql.