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/explore/search.tmpl

@@ -1,4 +1,4 @@
-<div class="ui small secondary filter menu tw-content-center gt-mx-0">
+<div class="ui small secondary filter menu tw-items-center gt-mx-0">
 	<form class="ui form ignore-dirty tw-flex-1">
 		{{if .PageIsExploreUsers}}
 			{{template "shared/search/combo" dict "Value" .Keyword "Placeholder" (ctx.Locale.Tr "search.user_kind")}}

templates/explore/user_list.tmpl

@@ -1,6 +1,6 @@
 <div class="flex-list">
 	{{range .Users}}
-		<div class="flex-item tw-content-center">
+		<div class="flex-item tw-items-center">
 			<div class="flex-item-leading">
 				{{ctx.AvatarUtils.Avatar . 48}}
 			</div>