Use post_date instead of post_date_gmt

[Caitboard]

Issue link

https://app.asana.com/0/1199679645691933/1204547940254524/f

Description

We use post_date_gmt to show the date on an article, this converts the time to GMT, resulting in the wrong date/time being shown for the post publication.
We want to use the date/time in the timezone where we have published the article, so we use post_date instead.

How Has This Been Tested?

• Set up a test post with the published time 12:30 AM. Check the article date in the frontend.
• Run this fix
• See test post again and see that the correct date is now displayed.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.