`<rh-button>` disabled tokens

[bennypowers]

Disabled button on light should be using --rh-color-gray-50, but there’s no semantic token for this. Should probably add disabled color tokens.

Originally posted by @marionnegp in https://github.com/RedHat-UX/red-hat-design-system/issues/1852

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It wasn't clear to me from looking at the figma file exactly what this refers to.

[coreyvickery]

If I just need to provide the color tokens for this, here they are.

Light theme

• Text = --rh-color-gray-60
• Surface = --rh-color-gray-30

Dark theme

• Text = --rh-color-gray-40
• Surface = --rh-color-gray-70

[markcaron]

Potentially create interactive-disabled tokens