[Reader IA] Fix ReaderPostTable sorting date as string

[RenanLukas]

Fixes #19897

I've changed the methods that were previously sorting dates as strings to use datetime in ReaderPostTable (see issue for context). In all scenarios the sorting was already working as expected, but this is only true because the posts have the same timezone. It's a weak solution IMHO because if the timezone changes it will break the sorting everywhere. The goal of this PR is to avoid having these problems in the future (like we had with #19881).

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To Test:

Basically Reader feeds have to continue working as expected.

• Instal JP and sign in;
• Open "Reader";
• Verify "Discover", "Subscriptions", "Saved", "Liked", "Automattic" and custom lists are working as expected (specially the sorting). Please note that not all feeds have the same sorting based on date_publishing (see here);
• Open any blog details;
• Verify the posts list is working as expected (specially the sorting);
• Open any tag;
• Verify the posts list is working as expected (specially the sorting).

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Regression Notes

1. Potential unintended areas of impact

   • Reader feeds

2. What I did to test those areas of impact (or what existing automated tests I relied on)

   • Manual testing

3. What automated tests I added (or what prevented me from doing so)

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PR Submission Checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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UI Changes Testing Checklist:

• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• Talkback.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• Large and small screen sizes. (Tablet and smaller phones)
• Multi-tasking: Split screen and Pop-up view. (Android 10 or higher)

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr19949-6ecdb71
	Commit	6ecdb71
	Direct Download	jetpack-prototype-build-pr19949-6ecdb71.apk

Note: Google Login is not supported on these builds.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr19949-6ecdb71
	Commit	6ecdb71
	Direct Download	wordpress-prototype-build-pr19949-6ecdb71.apk

Note: Google Login is not supported on these builds.

[peril-wordpress-mobile]

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[dangermattic]

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[thomashorta]

The code looks good to me and changes seem to be working as expected. Thanks for this PR!