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/forks.tmpl

@@ -6,7 +6,7 @@
 			{{ctx.Locale.Tr "repo.forks"}}
 		</h2>
 		{{range .Forks}}
-			<div class="tw-flex tw-content-center gt-py-3">
+			<div class="tw-flex tw-items-center gt-py-3">
 				<span class="gt-mr-2">{{ctx.AvatarUtils.Avatar .Owner}}</span>
 				<a href="{{.Owner.HomeLink}}">{{.Owner.Name}}</a> / <a href="{{.Link}}">{{.Name}}</a>
 			</div>