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/heatmap.js

@@ -3,7 +3,7 @@ import ActivityHeatmap from '../components/ActivityHeatmap.vue';
 import {translateMonth, translateDay} from '../utils.js';
 
 export function initHeatmap() {
-  const el = document.getElementById('user-heatmap');
+  const el = document.querySelector('#user-heatmap');
   if (!el) return;
 
   try {