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/commits.tmpl

@@ -4,7 +4,7 @@
 	<div class="ui container">
 		{{template "repo/sub_menu" .}}
 		<div class="repo-button-row">
-			<div class="tw-flex tw-content-center">
+			<div class="tw-flex tw-items-center">
 				{{template "repo/branch_dropdown" dict "root" . "ContainerClasses" "gt-mr-2"}}
 				<a href="{{.RepoLink}}/graph" class="ui basic small compact button">
 					{{svg "octicon-git-branch"}}