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>

package.json

@@ -23,9 +23,8 @@
     "fomantic-ui": "2.8.4",
     "highlight.js": "9.18.1",
     "imports-loader": "0.8.0",
-    "jquery": "3.4.1",
+    "jquery": "3.5.0",
     "jquery-datetimepicker": "2.5.21",
-    "jquery-migrate": "3.1.0",
     "jquery.are-you-sure": "1.9.0",
     "less-loader": "5.0.0",
     "mini-css-extract-plugin": "0.9.0",