Bugfix: Set ?readonly only on color swatches, and not on the individual swatch

[iOvergaard]

Description

fix: only set the ?readonly attribute on the swatches element since it handles setting it on the colors afterwards based on this comment: #2451 (comment)

CC @bjarnef - is this reasonable?

[bjarnef]

@iOvergaard yes :)

I think we need to remove disabled and readonly attributes as well in case developer actually set disabled or readonly on nested <uui-color-swatch> element. I think <uui-color-swatches> then decide:
https://github.com/umbraco/Umbraco.UI/blob/v1/contrib/packages/uui-color-swatches/lib/uui-color-swatches.element.ts#L91-L100

I haven't checked that scenario though, but the swatches would probably be rendered as is.

[iOvergaard]

Maybe yeah, @bjarnef. Thanks for the glance, though!