RSS: Correctly output 'datetime' string

[Mamaduka]

What?

Fixes #47000.

PR fixes the datetime output for the RSS block.

Why?

The date_i18n requires the correct format string, but the get_option( 'c' ) doesn't exist and returns false, preventing the date output.

How?

I've updated to code, and now the format is passed directly.

Testing Instructions

1. Open a Post or Page.
2. Insert an RSS Block.
3. Set feed URL. Example - https://make.wordpress.org/core
4. Toggle "Display date" from the settings.
5. Confirm that the datetime attribute is correctly set. It's easier to check this by DevTools Inspector.

Screenshots or screencast

Before

After

[Soean]

Oh, that's been wrong for quite a long time. Thanks for the fix!

[github-actions]

Flaky tests detected in 66359fc.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/3883583547
📝 Reported issues:

• [Flaky Test] should change image when user is changed #43593 in /test/e2e/specs/editor/blocks/avatar.spec.js