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## Purpose of the pull request

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## Brief change log

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  - *Add maven-checkstyle-plugin to root pom.xml*
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## Verify this pull request

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This pull request is code cleanup without any test coverage.

*(or)*

This pull request is already covered by existing tests, such as *(please describe tests)*.

(or)

This change added tests and can be verified as follows:

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  - *Added dolphinscheduler-dao tests for end-to-end.*
  - *Added CronUtilsTest to verify the change.*
  - *Manually verified the change by testing locally.* -->
