ci: add resource group and environment to deploy job

This commit adds basic deployment safety and stability improvements.

The resource group acts as mutex, preventing multiple deploy jobs to run simultaneously. This prevents race conditions e.g. when multiple MRs are merged (and thus multiple deploy pipelines run in parallel).

The environment makes rollbacks faster in case a bad state was deployed. Furthermore, it allows deploy secrets to be specified as protected environment variables, making them only available in the production environment and not other CI jobs.

Important note: Before merging, a new environment production needs to be added manually in the deployments section of this GitLab project!