Fix: Add version 3 migration for theme.json in post-featured-image block

[Sourav61]

What?

Closes #69769
This PR fixes the issue related to the migration handling for theme.json versions, specifically by adding support for version 3 of the theme.json configuration. The changes ensure that the block correctly handles the aspect ratio and height attributes during migration.

Why?

This PR is necessary because there is an issue with the migration process of theme.json versions 1 and 2 to version 3. Specifically, the migration logic was not handling version 3 correctly, leading to problems with setting the aspect ratio for blocks. This fix ensures that migration to version 3 works seamlessly.

How?

This PR addresses the issue by:

1. Adding a switch case to handle theme.json version 3 during migration.
2. Ensuring that the aspect ratio and height attributes are migrated properly from version 2 to version 3.
3. Preventing issues where blocks would fail to render correctly when migrating from version 2 to 3

Testing Instructions

1. Check the migration logic by using a theme.json with version 2.
2. Ensure the migration to version 3 works properly, and aspect ratio and height attributes are applied correctly.
3. Verify that blocks using the post-featured-image block are rendered correctly after the migration.