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/org/settings/labels.tmpl

@@ -1,6 +1,6 @@
 {{template "org/settings/layout_head" (dict "ctxData" . "pageClass" "organization settings labels")}}
 				<div class="org-setting-content">
-					<div class="tw-flex tw-content-center">
+					<div class="tw-flex tw-items-center">
 						<div class="tw-flex-1">
 							{{ctx.Locale.Tr "org.settings.labels_desc"}}
 						</div>