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/actions/list.tmpl

@@ -25,7 +25,7 @@
 				</div>
 			</div>
 			<div class="twelve wide column content">
-				<div class="ui secondary filter menu tw-justify-end tw-flex tw-content-center">
+				<div class="ui secondary filter menu tw-justify-end tw-flex tw-items-center">
 					<!-- Actor -->
 					<div class="ui{{if not .Actors}} disabled{{end}} dropdown jump item">
 						<span class="text">{{ctx.Locale.Tr "actions.runs.actor"}}</span>

templates/repo/actions/runs_list.tmpl

@@ -6,7 +6,7 @@
 	</div>
 	{{end}}
 	{{range .Runs}}
-		<div class="flex-item tw-content-center">
+		<div class="flex-item tw-items-center">
 			<div class="flex-item-leading">
 				{{template "repo/actions/status" (dict "status" .Status.String)}}
 			</div>

templates/repo/actions/status.tmpl

@@ -12,7 +12,7 @@
 {{- $className = .className -}}
 {{- end -}}
 
-<span class="tw-flex tw-content-center" data-tooltip-content="{{ctx.Locale.Tr (printf "actions.status.%s" .status)}}">
+<span class="tw-flex tw-items-center" data-tooltip-content="{{ctx.Locale.Tr (printf "actions.status.%s" .status)}}">
 {{if eq .status "success"}}
 	{{svg "octicon-check-circle-fill" $size (printf "text green %s" $className)}}
 {{else if eq .status "skipped"}}