Update disabled checkbox cursor style

[amitraj2203]

What?

Fixes: #62349

Why?

In the Site Editor's data views, checkboxes remain visible even when items are unselectable, causing confusion and suggesting interaction.

How?

Changing the cursor to not allowed for unselectable items in the Site Editor improves clarity and usability.

Testing Instructions

1. Open site editor.
2. Select Templates.
3. Try to select the unselectable templates.

Screenshots or screencast

Screen.Recording.2024-06-07.at.4.29.18.PM.mov

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Co-authored-by: annezazu <annezazu@git.wordpress.org>

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[jorgefilipecosta]

The change looks good code-wise, but I'm going to cc: @WordPress/gutenberg-design so designers can have a look.

[jameskoster]

I think this is reasonable. Visually there may not be enough contrast between enabled/disabled checkboxes, and the cursor helps mouse users. The question I have is whether the cursor should be applied holistically across all disabled elements.