Switch to using the Snakemake builtin slurm executor rather than relying on profiles

[holtgrewe]

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

Describe the solution you'd like
The Snakemake Slurm profile is not 100% reliable. Recent versions of Snakemake have a builtin executor that is reliable.

Describe alternatives you've considered
Switch to using this builtin executor. We might have to properly use resources with mem_gb consistently.

Additional context

• Some examples of using resources in varfish-db-downloader