Update Hugo.gitlab-ci.yml template

What does this MR do?

This PR changes from the Hugo page rendering image dettmering/hugo-build to registry.gitlab.com/pages/hugo:latest. The latter uses a more recent version of Hugo and thereby allows to use more recent themes.

Does this MR meet the acceptance criteria?

Conformity

This just changes a template and not any actual code.

Availability and Testing

I've tested the CI template and it works just fine. No cross browser testing should be required.

Security

As this uses an official docker image instead of the one from dettermering it is likely even more secure.