Fix undefined share object during expirationDate update

In 04e8733 a regression was introduced.

`this` means nothing inside arrow functions, hence this.share was referring to

an undefined object and so all attempted updates for expiration dates would fail.

Resolves : #43256

Signed-off-by: fenn-cs <fenn25.fn@gmail.com>

[skip ci]

apps/files_sharing/src/mixins/SharesMixin.js

@@ -221,7 +221,7 @@ export default {
 		 *
 		 * @param {Date} date
 		 */
-		onExpirationChange: debounce((date) => {
+		onExpirationChange: debounce(function(date) {
 			this.share.expireDate = this.formatDateToString(new Date(date))
 		}, 500),
 		/**