Fix incorrect tailwind migration (#30007)

Fixes https://github.com/go-gitea/gitea/issues/30005. Regression from
https://github.com/go-gitea/gitea/pull/29945.

There was only once instance of `tw-content-center` before that PR, so I
just ran below command and reverted that one instance.

```sh
perl -p -i -e 's#tw-content-center#tw-items-center#g' web_src/js/**/* templates/**/* models/**/* tests/**/*
```

web_src/js/features/repo-issue-content.js

@@ -16,7 +16,7 @@ function showContentHistoryDetail(issueBaseUrl, commentId, historyId, itemTitleH
   $dialog = $(`
 <div class="ui modal content-history-detail-dialog">
   ${svg('octicon-x', 16, 'close icon inside')}
-  <div class="header tw-flex tw-content-center tw-justify-between">
+  <div class="header tw-flex tw-items-center tw-justify-between">
     <div>${itemTitleHtml}</div>
     <div class="ui dropdown dialog-header-options gt-mr-5 gt-hidden">
       ${i18nTextOptions}