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/shared/searchbottom.tmpl

@@ -1,5 +1,5 @@
-<div class="ui bottom attached table segment tw-flex tw-content-center tw-justify-between">
-		<div class="tw-flex tw-content-center gt-ml-4">
+<div class="ui bottom attached table segment tw-flex tw-items-center tw-justify-between">
+		<div class="tw-flex tw-items-center gt-ml-4">
 			{{if .result.Language}}
 					<i class="color-icon gt-mr-3" style="background-color: {{.result.Color}}"></i>{{.result.Language}}
 			{{end}}

templates/shared/secrets/add_list.tmpl

@@ -14,7 +14,7 @@
 	{{if .Secrets}}
 	<div class="flex-list">
 		{{range .Secrets}}
-		<div class="flex-item tw-content-center">
+		<div class="flex-item tw-items-center">
 			<div class="flex-item-leading">
 				{{svg "octicon-key" 32}}
 			</div>

templates/shared/user/org_profile_avatar.tmpl

@@ -2,7 +2,7 @@
 	<div class="ui container">
 		<div class="ui vertically grid head">
 			<div class="column">
-				<div class="ui header tw-flex tw-content-center gt-word-break">
+				<div class="ui header tw-flex tw-items-center gt-word-break">
 					{{ctx.AvatarUtils.Avatar . 100}}
 					<span class="text thin grey"><a href="{{.HomeLink}}">{{.DisplayName}}</a></span>
 					<span class="org-visibility">

templates/shared/variables/variable_list.tmpl

@@ -16,7 +16,7 @@
 	{{if .Variables}}
 	<div class="flex-list">
 		{{range .Variables}}
-		<div class="flex-item tw-content-center">
+		<div class="flex-item tw-items-center">
 			<div class="flex-item-leading">
 				{{svg "octicon-pencil" 32}}
 			</div>