Fix port validations in .gitlab-webide.yml

What does this MR do?

This is the CE port of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/13846

Refs https://gitlab.com/gitlab-org/gitlab-ee/issues/11886

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content