Administration: Fix date column showing post time instead of modified time

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/63835

When viewing the admin post/page listing, the "Date" column shows the "Last Modified" label for unpublished private posts, but incorrectly displays the post creation time instead of the actual modification time.

This patch fixes the issue by:

• Determining upfront whether to show post time or modified time based on post status
• Using get_the_modified_time() and get_post_modified_time() for unpublished posts
• Maintaining consistent timestamp calculations for both display and time difference

All other statuses now correctly show the modified time when labeled "Last Modified".

[github-actions]

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