Update migrate.sh with official proposal number

migrate.sh

@@ -63,7 +63,7 @@ LINE=$(sed -n '/Test-only packages can have anything they want/=' $GOROOT/src/go
 sed -i "$((LINE+1)) i\\\tFMT, compress/gzip, embed, encoding/binary < encoding/json/internal/jsontest;" $GOROOT/src/go/build/deps_test.go
 
 # Adjust for newly added API.
-ISSUE=63397 # TODO: Replace with formal proposal issue for encoding/json/v2
+ISSUE=71497
 FILE=$(cd $GOROOT/api; ls -v | tail -n 1)
 echo "pkg encoding/json, func CallMethodsWithLegacySemantics(bool) jsonopts.Options #$ISSUE" >> $GOROOT/api/$FILE
 echo "pkg encoding/json, func DefaultOptionsV1() jsonopts.Options #$ISSUE" >> $GOROOT/api/$FILE