Set version to 2.6.0.

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013436
Japanese translation updated by Go MAEDA (#18069)

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extra : convert_revision : svn%3Ae93f8b46-1217-0410-a6f0-8f06a7374b81/trunk%4013439