Remove jQuery from repo wiki creation page (#29271)

- Switched to plain JavaScript
- Tested the wiki creation form functionality and it works as before

# Demo using JavaScript without jQuery

![action](https://github.com/go-gitea/gitea/assets/20454870/2dfc95fd-40cc-4ffb-9ae6-50f798fddd67)

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Signed-off-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: silverwind <me@silverwind.io>

web_src/js/features/repo-diff.js

@@ -47,8 +47,8 @@ function initRepoDiffConversationForm() {
     e.preventDefault();
 
     const $form = $(e.target);
-    const $textArea = $form.find('textarea');
-    if (!validateTextareaNonEmpty($textArea)) {
+    const textArea = e.target.querySelector('textarea');
+    if (!validateTextareaNonEmpty(textArea)) {
       return;
     }