Remove possibility for listing multiple GenotypeInfo for "genotype"

That would avoid Union of a list with non-multivalued identifier. And we already have
data with string value describing the genotype -- it is not really an identifier!!!

dandischema/models.py

@@ -1373,7 +1373,7 @@ class Participant(DandiBaseModel):
         "available. (e.g. from OBI)",
         json_schema_extra={"nskey": "dandi"},
     )
-    genotype: Optional[Union[List[GenotypeInfo], Identifier]] = Field(
+    genotype: Optional[Union[GenotypeInfo, Identifier]] = Field(
         None,
         description="Genotype descriptor of participant or subject if available",
         json_schema_extra={"nskey": "dandi"},