Upgrade jQuery to 3.5.0, remove jQuery-Migrate, fix deprecations

- 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: #9372

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",

public/vendor/librejs.html

@@ -18,12 +18,7 @@
         <tr>
           <td><a href="../js/jquery.js">jQuery</a></td>
           <td><a href="https://jquery.org/license/">MIT</a></td>
-          <td><a href="https://code.jquery.com/jquery-3.4.1.min.js">jquery-3.4.1.min.js</a></td>
-        </tr>
-        <tr>
-          <td><a href="../js/jquery.js">jQuery Migrate</a></td>
-          <td><a href="https://jquery.org/license/">MIT</a></td>
-          <td><a href="https://code.jquery.com/jquery-migrate-3.1.0.min.js">jquery-migrate-3.1.0.min.js</a></td>
+          <td><a href="https://code.jquery.com/jquery-3.5.0.min.js">jquery-3.5.0.min.js</a></td>
         </tr>
         <tr>
           <td><a href="../fomantic/semantic.min.js">semantic.min.js</a></td>