Fix camelCase and cross-component class name

[johngodley]

Fixes camel case typo in tabIndex in the block comparison modal, and use a class name local to the component.

This fixes the points raised #7995 (comment)

How has this been tested?

Existing tests updated for new names.

Trigger an invalid block and pick 'compare block conversion' from the ellipsis menu. Verify that the modal title looks the same:

As it looked before:

Types of changes

Minor class and attribute change

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[talldan]

👍