services: colord: Deprecate 'colord-service' procedure.

* gnu/services/desktop.scm (colord-service-type)[default-value]: New field.
(colord-service): Define with 'define-deprecated'.
(%desktop-services): Use 'colord-service-type' instead of 'colord-service'.
* doc/guix.texi (Desktop Services): Adjust accordingly.