Switch sgn GAF source to goex mirror

For #2652. Switch GAF source URL for sgn from solgenomics.net to the
GO mirror of goex (SOLLC_4081_UP000004994.gaf.gz), with mirror_of
pointing to the EBI goex source.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kltm

metadata/datasets/sgn.yaml

@@ -16,7 +16,8 @@ datasets:
    dataset: sgn
    submitter: sgn
    compression: gzip
-   source: https://solgenomics.net/ftp/ontology/GO/gene_association.sgn.gz
+   source: https://mirror.geneontology.io/SOLLC_4081_UP000004994.gaf.gz
+   mirror_of: https://ftp.ebi.ac.uk/pub/contrib/goa/goex/current/gaf/SOLLC_4081_UP000004994.gaf.gz
    entity_type: gene
    status: active
    species_code: Solanaceae