Replace commons.lang with plain java

[mawinter69]

commons-lang2 is provided by core. As it is deprecated use plain java.

Testing done

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[mawinter69]

The version 6.2108 is buggy as it would produce Java 21 bytecode

[alecharp]

If a release of the plugin could be done with this changeset, it would help getting jenkinsci/calendar-view-plugin working without commons-lang 2 (CronJobServiceTest not working with this pull request) and overall it would help make progress on jenkinsci/bom#6234 and jenkinsci/jenkins#26105. Thank you.