Now we need to redirect the former projects.spring.io/spring-security-kerberos page to the new one. In your project repository, check out the gh-pages branch and edit the index.html to make it look like [1] (again, adapt the project name to your case). You can delete all the other files. Commit and push!
Instructions from @bclozel:
[1] - https://github.com/spring-projects/spring-framework/blob/gh-pages/index.html