Make YAML linting more strict

## Context

We may want to make the YAML linting more strict, disallowing warnings emitted by `yamllint`. Additionally, we may want to run YAML linting for the yaml files which are part of the template management (e.g. GitHub actions for this repository).

Originally submitted in https://github.com/projectsyn/modulesync-control/pull/73
See also https://github.com/projectsyn/commodore/pull/468
