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/settings/branches.tmpl

@@ -41,7 +41,7 @@
 			<div class="ui attached segment">
 				<div class="flex-list">
 					{{range .ProtectedBranches}}
-						<div class="flex-item tw-content-center">
+						<div class="flex-item tw-items-center">
 							<div class="flex-item-main">
 								<div class="flex-item-title">
 									<div class="ui basic primary label">{{.RuleName}}</div>

templates/repo/settings/collaboration.tmpl

@@ -7,7 +7,7 @@
 		<div class="ui attached segment">
 			<div class="flex-list">
 				{{range .Collaborators}}
-					<div class="flex-item tw-content-center">
+					<div class="flex-item tw-items-center">
 						<div class="flex-item-leading">
 							<a href="{{.HomeLink}}">{{ctx.AvatarUtils.Avatar . 32}}</a>
 						</div>

templates/repo/settings/options.tmpl

@@ -837,7 +837,7 @@
 					</div>
 				</div>
 				{{if not .Repository.IsMirror}}
-					<div class="flex-item tw-content-center">
+					<div class="flex-item tw-items-center">
 						<div class="flex-item-main">
 							{{if .Repository.IsArchived}}
 								<div class="flex-item-title">{{ctx.Locale.Tr "repo.settings.unarchive.header"}}</div>