Remove --stats when we upgrade to Snakemake v8

[joverlee521]

Follow up to #451.

We are using the Snakemake --stats flag to get workflow stats for GISAID and GenBank ingest to help with #446. If/when we ever upgrade Snakemake to v8, the --stats flag is no longer supported and will need to be removed.

Upgrading Snakemake is dependent on updating the nextstrain docker-base image and then running the update-image workflow in this repo since the workflows are using the nextstrain/ncov-ingest Docker image.

Alternatively, we can pin Snakemake <8 in this repo, but we can make that decision later.

[corneliusroemer]

I don't see why we shouldn't support snakemake 8. I think it's bad we get stuck at old versions and can't use new features.

It's good that we are updating now but I agree that pinning should be easy workaround in case we don't have time to change the worklow immediately