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/wiki/pages.tmpl

@@ -2,7 +2,7 @@
 <div role="main" aria-label="{{.Title}}" class="page-content repository wiki pages">
 	{{template "repo/header" .}}
 	<div class="ui container">
-		<h2 class="ui header tw-flex tw-content-center tw-justify-between">
+		<h2 class="ui header tw-flex tw-items-center tw-justify-between">
 			<span>{{ctx.Locale.Tr "repo.wiki.pages"}}</span>
 			<span>
 				{{if and .CanWriteWiki (not .Repository.IsMirror)}}