feat(GlDatepicker): use variant prop on GlIcon for disabled control

mentioned in commit 989f7a31

mentioned in merge request status-page!903

mentioned in merge request gitlab!182887 (merged)

mentioned in merge request gitlab!183036 (closed)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!64 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4321 (merged)

mentioned in merge request gitlab-org/editor-extensions/gitlab-jetbrains-plugin!1551 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!367 (merged)