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/user/settings/applications_oauth2_list.tmpl

@@ -4,7 +4,7 @@
 			{{ctx.Locale.Tr "settings.oauth2_application_create_description"}}
 		</div>
 		{{range .Applications}}
-			<div class="flex-item tw-content-center">
+			<div class="flex-item tw-items-center">
 				<div class="flex-item-leading">
 					{{svg "octicon-apps" 32}}
 				</div>

templates/user/settings/security/openid.tmpl

@@ -7,7 +7,7 @@
 			{{ctx.Locale.Tr "settings.openid_desc"}}
 		</div>
 		{{range .OpenIDs}}
-			<div class="flex-item tw-content-center">
+			<div class="flex-item tw-items-center">
 				<div class="flex-item-leading">
 					{{svg "fontawesome-openid" 20}}
 				</div>