Use querySelector over alternative DOM methods (#31280)

As per
https://github.com/go-gitea/gitea/pull/30115#discussion_r1626060164,
prefer `querySelector` by enabling
[`unicorn/prefer-query-selector`](https://github.com/sindresorhus/eslint-plugin-unicorn/blob/main/docs/rules/prefer-query-selector.md)
and autofixing all except 10 issues.

According to
[this](https://old.reddit.com/r/learnjavascript/comments/i0f5o8/performance_of_getelementbyid_vs_queryselector/),
querySelector may be faster as well, so it's a win-win.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

web_src/js/features/repo-issue-pr-form.js

@@ -2,7 +2,7 @@ import {createApp} from 'vue';
 import PullRequestMergeForm from '../components/PullRequestMergeForm.vue';
 
 export function initRepoPullRequestMergeForm() {
-  const el = document.getElementById('pull-request-merge-form');
+  const el = document.querySelector('#pull-request-merge-form');
   if (!el) return;
 
   const view = createApp(PullRequestMergeForm);