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/copycontent.js

@@ -6,7 +6,7 @@ import {GET} from '../modules/fetch.js';
 const {i18n} = window.config;
 
 export function initCopyContent() {
-  const btn = document.getElementById('copy-content');
+  const btn = document.querySelector('#copy-content');
   if (!btn || btn.classList.contains('disabled')) return;
 
   btn.addEventListener('click', async () => {