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/webhook/new.tmpl

@@ -1,7 +1,7 @@
 <h4 class="ui top attached header">
 	{{.CustomHeaderTitle}}
 	<div class="ui right type dropdown">
-		<div class="text tw-flex tw-content-center">
+		<div class="text tw-flex tw-items-center">
 			{{template "shared/webhook/icon" (dict "Size" 20 "HookType" .ctxData.HookType)}}
 			{{ctx.Locale.Tr (print "repo.settings.web_hook_name_" .ctxData.HookType)}}
 		</div>