fix: make datepicker more accessible (#3343)
fix: make datepicker more accessible (#3343)

A finding of an a11y test on a project, where the Datepicker is used, was that the proposed columnheader role is missing, thus making the datepicker not accessible for blind users.
ng-bootstrap/ng-bootstrap,ng-bootstrap/ng-bootstrap,fbasso/ng-bootstrap,fbasso/ng-bootstrap,benouat/ng-bootstrap,fbasso/ng-bootstrap,maxokorokov/core,jnizet/ng-bootstrap,maxokorokov/core,maxokorokov/core,benouat/ng-bootstrap,jnizet/ng-bootstrap,maxokorokov/core,benouat/ng-bootstrap,ng-bootstrap/ng-bootstrap,jnizet/ng-bootstrap,jnizet/ng-bootstrap,benouat/ng-bootstrap,ng-bootstrap/ng-bootstrap,fbasso/ng-bootstrap