fix(GlDatepicker): Add border radius to icon to prevent Safari issue

mentioned in commit 77dec9f8

mentioned in merge request status-page!903

mentioned in merge request gitlab!163601 (merged)

mentioned in merge request gitlab!163047 (closed)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4016 (merged)

mentioned in merge request gitlab-org/gitlab-services/design.gitlab.com!4017 (merged)

mentioned in merge request gitlab!163882 (closed)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!342 (merged)

mentioned in merge request gitlab-org/frontend/playground/accessibility-scanner!49 (merged)

mentioned in merge request gitlab-org/frontend/pajamas-adoption-scanner!343 (merged)