Fix MenuItem disabled colors/styling

[brandongregoryscott]

Overview Resolves #1497

Fixes up the few styling discrepancies that were noted, which should be testable in the existing Storybook story

• Cursor is now actually not-allowed (the pointerEvents: 'none' style was preventing this)
• Secondary text is always muted
• Primary text is muted when disabled, or the intent color
• Icon color is muted when disabled, or the intent color

Screenshots (if applicable)

Documentation

• [ ] Updated Typescript types and/or component PropTypes
• [ ] Added / modified component docs
• [ ] Added / modified Storybook stories

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