List Table: Fix incorrect use of row headers in post list tables

[himanshupathak95]

Trac ticket: https://core.trac.wordpress.org/ticket/32892

Changes the semantic structure so that:

• Checkbox columns use <td> instead of <th scope="row">
• Primary content columns (like post titles) use <th scope="row"> as proper row headers

This ensures screen readers announce meaningful content (post titles, usernames, etc.) as row headers instead of "Select [item]" from checkboxes.

[himanshupathak95]

Other list tables, such as the Users list and multi-site tables, are yet to be updated. Additionally, the styling has been affected, which I will address in the subsequent commits.

[github-actions]

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