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/issue/milestone_issues.tmpl

@@ -29,7 +29,7 @@
 		<div class="tw-flex tw-flex-col gt-gap-3">
 			<progress class="milestone-progress-big" value="{{.Milestone.Completeness}}" max="100"></progress>
 			<div class="tw-flex gt-gap-4">
-				<div classs="tw-flex tw-content-center">
+				<div classs="tw-flex tw-items-center">
 					{{$closedDate:= TimeSinceUnix .Milestone.ClosedDateUnix ctx.Locale}}
 					{{if .IsClosed}}
 						{{svg "octicon-clock"}} {{ctx.Locale.Tr "repo.milestones.closed" $closedDate}}