Adjust gene maps in flu datasets to v2

[ivan-aksamentov]

Flu datasets have their gene map attribute keys and values separated with spaces. But in GFF3 spec the separator is = instead. Nextclade v2 will not find any genes silently, unless this is fixed. Nextclade v1 supports both, so this change is not breaking.

SC2 and Monkeypox datasets seems to be using = already, but double checking all the dataset won't hurt.