Replace jquery-datetimepicker with native date input (#11684)

This removes the jQuery plugin as well as the associated config options.
Native input[type=date] does not require a language attribute as it is
localized by default, except for the placeholder attribute for which I
currently piggy-back the repo.issues.due_date_form localization option.

Implementation should pretty much match GH. Of note is that Safari does
not provide a UI for this input type, but I don't think providing one is
neccessary and GH did not bother either.

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

web_src/js/features/datetimepicker.js

@@ -1,8 +0,0 @@
-export default async function initDateTimePicker(locale) {
-  await Promise.all([
-    import(/* webpackChunkName: "datetimepicker" */'jquery-datetimepicker'),
-    import(/* webpackChunkName: "datetimepicker" */'jquery-datetimepicker/build/jquery.datetimepicker.min.css'),
-  ]);
-
-  $.datetimepicker.setLocale(locale);
-}

web_src/js/index.js

@@ -12,7 +12,6 @@ import initContextPopups from './features/contextpopup.js';
 import initGitGraph from './features/gitgraph.js';
 import initClipboard from './features/clipboard.js';
 import initUserHeatmap from './features/userheatmap.js';
-import initDateTimePicker from './features/datetimepicker.js';
 import initServiceWorker from './features/serviceworker.js';
 import attachTribute from './features/tribute.js';
 import createDropzone from './features/dropzone.js';
@@ -755,18 +754,6 @@ async function initRepository() {
 
   // Milestones
   if ($('.repository.new.milestone').length > 0) {
-    const $datepicker = $('.milestone.datepicker');
-
-    await initDateTimePicker($datepicker.data('lang'));
-
-    $datepicker.datetimepicker({
-      inline: true,
-      timepicker: false,
-      startDate: $datepicker.data('start-date'),
-      onSelectDate(date) {
-        $('#deadline').val(date.toISOString().substring(0, 10));
-      },
-    });
     $('#clear-date').on('click', () => {
       $('#deadline').val('');
       return false;