Post Date: Add border block support

[akasunil]

What?

Add border block support to the Post Date block.

Part of #43247

Why?

Post Date block is missing border support.

How?

Adds the border block support in block.json

Testing Instructions

• Go to Global Styles setting ( under appearance > editor > styles > edit styles > blocks )
• Make sure that Post Date block's border is configurable via Global Styles
• Verify that Global Styles are applied correctly in the editor and frontend
• Edit template/page, Add Post Date block and Apply the border styles
• Verify that block styles take precedence over global styles
• Verify that block borders display correctly in both the editor and frontend

Please refer to the attached video for more information.

Screenshots or screencast

Blog-Home-.-Template-.-gutenberg-.-Editor-.-WordPress.12.webm

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: akasunil <sunil25393@git.wordpress.org>
Co-authored-by: aaronrobertshaw <aaronrobertshaw@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[aaronrobertshaw]

Thanks for the continued effort on adopting block supports more broadly 👍

✅ Global styles display correctly
✅ Block instance styles override global styles
✅ Styling is consistent between editor and frontend
✅ Box-sizing was already set to border-box for when parent enforces an explicit width

This looks good to go to me 🚀