Update the Cover Block placeholder for consistency with the Image Block

[priethor]

What problem does this address?

Since the Image block placeholder state was improved in #43180, the Cover block placeholder is not consistent with the Image block's anymore, despite having a very similar interface when focused during their setup stage:

Grabacion.de.pantalla.2022-11-24.a.las.17.21.59.mov

What is your proposed solution?

The Cover Block placeholder state could show a placeholder image instead of the setup options, similar to how the Post Featured Image block and the Image block's placeholders work.

[richtabor]

Came here to report this inconsistency as well.

[richtabor]

The core/media-text block is also behind:

[jordesign]

Worth noting that the 'Insert from URL' could be an interesting addition here as well.
#47355