Fix the frame.append warning

It's probably already deprecated in current `pandas` versions (we pin it)
```
/miniconda3/envs/spacemake/lib/python3.10/site-packages/spacemake/snakemake/main.smk:691:
FutureWarning: The frame.append method is deprecated and will be removed from pandas in
a future version. Use pandas.concat instead.
```