Upgrade jQuery to 3.5.0, remove jQuery-Migrate, fix deprecations (#11055)

- Removed jQuery-Migrate as it seems to no longer be needed.
- Removed a dead code section.
- Fixed some deprecations detected by jQuery-Migrate.

Ref: https://jquery.com/upgrade-guide/3.5/
Ref: https://github.com/gogs/gogs/search?q=repo-name-change-prompt
Ref: https://github.com/go-gitea/gitea/search?q=repo-name-change-prompt
Fixes: https://github.com/go-gitea/gitea/issues/9372

Co-authored-by: zeripath <art27@cantab.net>

web_src/js/jquery.js

@@ -1,5 +1,3 @@
 import $ from 'jquery';
-import 'jquery-migrate';
 
-$.migrateMute = true;
 window.$ = window.jQuery = $;