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/dashboard/milestones.tmpl

@@ -79,7 +79,7 @@
 									{{svg "octicon-milestone" 16}}
 									<a class="muted" href="{{.Repo.Link}}/milestone/{{.ID}}">{{.Name}}</a>
 								</h3>
-								<div class="tw-flex tw-content-center">
+								<div class="tw-flex tw-items-center">
 									<span class="gt-mr-3">{{.Completeness}}%</span>
 									<progress value="{{.Completeness}}" max="100"></progress>
 								</div>