Use Temporal.PlainDate for absolute dates (#29804) (#29808)

Backport https://github.com/go-gitea/gitea/pull/29804.

Use the upcoming
[Temporal.PlainDate](https://tc39.es/proposal-temporal/docs/plaindate.html)
via polyfill. If there is any remaining bugs in `<absolute-date>` this
will iron them out. I opted for the lightweight polyfill because both
seem to achieve our goal of localizeable absolute dates.

- With
[`@js-temporal/polyfill`](https://www.npmjs.com/package/@js-temporal/polyfill)
chunk size goes from 81.4 KiB to 274 KiB
- With
[`temporal-polyfill`](https://www.npmjs.com/package/temporal-polyfill)
chunk size goes from 81.4 KiB to 142 KiB

Also see [this
table](https://github.com/fullcalendar/temporal-polyfill?tab=readme-ov-file#comparison-with-js-temporalpolyfill)
for more comparisons of these polyfills. Soon there will be
[treeshakable
API](https://github.com/fullcalendar/temporal-polyfill?tab=readme-ov-file#tree-shakable-api)
as well which will further reduce size.

web_src/js/webcomponents/GiteaAbsoluteDate.test.js

@@ -0,0 +1,15 @@
+import {toAbsoluteLocaleDate} from './GiteaAbsoluteDate.js';
+
+test('toAbsoluteLocaleDate', () => {
+  expect(toAbsoluteLocaleDate('2024-03-15', 'en-US', {
+    year: 'numeric',
+    month: 'long',
+    day: 'numeric',
+  })).toEqual('March 15, 2024');
+
+  expect(toAbsoluteLocaleDate('2024-03-15', 'de-DE', {
+    year: 'numeric',
+    month: 'long',
+    day: 'numeric',
+  })).toEqual('15. März 2024');
+});