Part of https://core.trac.wordpress.org/ticket/53175

This ports this PR WordPress/gutenberg#33280 The JS changes in that PR will be ported in the next package release and both are independent (they fix two related but separate things).

How to test

• Add these contents to a file named theme.json in the TwentyTwentyOne theme (or add the same contents to any theme with theme.json support):

{
  "version": 1,
  "settings": {
    "color": {
      "duotone": []
    },
    "spacing": {
      "units": []
    }
  }
}

• Open the post editor.
• Add a cover block. Go to the inspector and verify that the padding control only show px units. Before this change, it'll show all units.
• Add an image block. Open the duotone control from the block toolbar. The expected result is that it doesn't show any duotone colors. Before this change, it'll still show all colors.