Administration: Remove not-allowed cursor from button color mixins

[SainathPoojary]

Removes cursor: not-allowed from the primary, secondary, and tertiary button mixins. This lets disabled buttons use the existing default cursor styles for consistency across the admin.

Trac ticket: https://core.trac.wordpress.org/ticket/66251

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Props sainathpoojary, afercia, dhruvang21.

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[afercia]

I pushed a commit to remove the remaining 3 occurrences of cursor: not-allowed;. They are all very edge cases and introduce unnecessary inconsistency anyways.

1
In the Classic Widgets page, only the 'Custom HTML` widget used it when the button so save is disabled:

2
In the Theme / Plugin file editors. It was used on 'Update File' button when disabled:

3
In the Customizer, when reordering menus or widgets, it was used for the 'Add Items' / 'Add Widgets' button. This didn;t work anyways becayse pointer-events: none; already resets the cursor style:

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 64229
GitHub commit: 36e7c85

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.