Migrate _download.js to use Bootstrap5 modals

[nboisteault]

Code should be :

    layersadded: function() {
        var html = '<div class="modal-dialog modal-dialog-scrollable"><div class="modal-content">';
        html+= '<div class="modal-header"><h3>Welcome on this map</h3><button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button></div>';

        html+= '<div class="modal-body">';
        html+= document.getElementById('metadata')?.innerHTML;
        html+= '</div>';

        html+= '<div class="modal-footer"><button type="button" class="btn btn-default" data-bs-dismiss="modal">Ok</button></div>';

        html += '</div></div>'

        const modalElt = document.getElementById('lizmap-modal');
        modalElt.innerHTML = html;
        const myModal = new bootstrap.Modal(modalElt);
        myModal.show();
    }