Fix port validation in .gitlab-webide.yml

What does this MR do?

Fixes a bug where the port wasn't validated properly when it was inside the services section in the .gitlab-webide.yml.

CE Port in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29016

Closes #11886 (closed).

Does this MR meet the acceptance criteria?

Conformity

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