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/admin/emails/list.tmpl

@@ -4,7 +4,7 @@
 			{{ctx.Locale.Tr "admin.emails.email_manage_panel"}} ({{ctx.Locale.Tr "admin.total" .Total}})
 		</h4>
 		<div class="ui attached segment">
-			<div class="ui secondary filter menu tw-content-center gt-mx-0">
+			<div class="ui secondary filter menu tw-items-center gt-mx-0">
 				<form class="ui form ignore-dirty tw-flex-1">
 					{{template "shared/search/combo" dict "Value" .Keyword}}
 				</form>

templates/admin/org/list.tmpl

@@ -7,7 +7,7 @@
 			</div>
 		</h4>
 		<div class="ui attached segment">
-			<div class="ui secondary filter menu tw-content-center gt-mx-0">
+			<div class="ui secondary filter menu tw-items-center gt-mx-0">
 				<form class="ui form ignore-dirty tw-flex-1">
 					{{template "shared/search/combo" dict "Value" .Keyword "Placeholder" (ctx.Locale.Tr "search.org_kind")}}
 				</form>

templates/admin/repo/unadopted.tmpl

@@ -20,7 +20,7 @@
 				{{if .Dirs}}
 					<div class="ui aligned divided list">
 						{{range $dirI, $dir := .Dirs}}
-							<div class="item tw-flex tw-content-center">
+							<div class="item tw-flex tw-items-center">
 								<span class="tw-flex-1"> {{svg "octicon-file-directory-fill"}} {{$dir}}</span>
 								<div>
 									<button class="ui button primary show-modal gt-p-3" data-modal="#adopt-unadopted-modal-{{$dirI}}">{{svg "octicon-plus"}} {{ctx.Locale.Tr "repo.adopt_preexisting_label"}}</button>

templates/admin/stacktrace-row.tmpl

@@ -1,5 +1,5 @@
 <div class="item">
-	<div class="tw-flex tw-content-center">
+	<div class="tw-flex tw-items-center">
 		<div class="icon gt-ml-3 gt-mr-3">
 			{{if eq .Process.Type "request"}}
 				{{svg "octicon-globe" 16}}
@@ -40,7 +40,7 @@
 						</summary>
 						<div class="list">
 							{{range .Entry}}
-								<div class="item tw-flex tw-content-center">
+								<div class="item tw-flex tw-items-center">
 									<span class="icon gt-mr-4">{{svg "octicon-dot-fill" 16}}</span>
 									<div class="content tw-flex-1">
 										<div class="header"><code>{{.Function}}</code></div>

templates/admin/stacktrace.tmpl

@@ -1,7 +1,7 @@
 {{template "admin/layout_head" (dict "ctxData" . "pageClass" "admin monitor")}}
 <div class="admin-setting-content">
 
-	<div class="tw-flex tw-content-center">
+	<div class="tw-flex tw-items-center">
 		<div class="tw-flex-1">
 			<div class="ui compact small menu">
 				<a class="{{if eq .ShowGoroutineList "process"}}active {{end}}item" href="?show=process">{{ctx.Locale.Tr "admin.monitor.process"}}</a>