CI: Make the h2spec workflow more consistent with the VTest workflow

CI: Make the h2spec workflow more consistent with the VTest workflow

This PR aims to make the workflow more consistent, by reusing the same wording
for the step names and the same commands to make it look like the vtest
workflow as much as possible.

It was renamed to compliance.yml to match the human readable name better. This
also allows to extend the workflow with other compliance tools later on, nicely
grouping those jobs together in a single file.

No functional changes have been made.