Update epubcheck-ruby requirement from 5.1.0.0 to 5.2.1.0

[dependabot[bot]]

Updates the requirements on epubcheck-ruby to permit the latest version.

Commits

• ca572bc Merge pull request #28 from takahashim/v5.2.1
• cf00b8f Update epubcheck 5.2.1
• 74e7f55 update jar 5.2.0 -> 5.2.1
• 32de721 Merge pull request #26 from takahashim/update-ga
• e9a78bb GA: support ruby 3.4
• fe255bd Merge pull request #25 from takahashim/update-520
• e194d55 fix GA
• fc2d1b2 bump up 5.2.0
• 70c9eea update jar 5.1.0 -> 5.2.0
• See full diff in compare view

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