Align theme of Haml form datepickers with gitlab-ui

What does this MR do and why?

We currently use two different themes for the gitlab_ui_datepicker (in our GitlabUiFormBuilder) and the GlDatepicker (gitlab-ui vue component).

We should use the same for both.

This switches the theme of the Haml datepicker to the latest.

Screenshots or screen recordings

How to set up and validate locally

All datepickers within the application should now have the same theme

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #370258