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/**/*
```

templates/repo/find/files.tmpl

@@ -2,7 +2,7 @@
 <div role="main" aria-label="{{.Title}}" class="page-content repository">
 	{{template "repo/header" .}}
 	<div class="ui container">
-		<div class="tw-flex tw-content-center">
+		<div class="tw-flex tw-items-center">
 			<a href="{{$.RepoLink}}">{{.RepoName}}</a>
 			<span class="gt-mx-3">/</span>
 			<div class="ui input tw-flex-1">