Add `disabled` prop to `GlDatepicker` story

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

See merge request gitlab-org/gitlab-ui!4993



Merged-by: Florie Guibert <fguibert@gitlab.com>
Approved-by: Jeremy Elder <jelder@gitlab.com>
Approved-by: Florie Guibert <fguibert@gitlab.com>
Co-authored-by: sdejonge <sdejonge@gitlab.com>

# [108.8.0](gitlab-org/gitlab-ui@v108.7.0...v108.8.0) (2025-02-27)

### Features

* **GlDatepicker:** use variant prop on GlIcon for disabled control ([f64fe843](gitlab-org/gitlab-ui@f64fe843))
* **GlLabel:** update scoped label title text color to text.color.strong ([6df75970](gitlab-org/gitlab-ui@6df75970))