CI: update test matrix with go-1.16
CI: update test matrix with go-1.16

and drop go-1.12

ploxiln/nsq,bitly/nsq,mreiferson/nsq,bitly/nsq,nsqio/nsq,mreiferson/nsq,nsqio/nsq,rpau/nsq,bitly/nsq,mreiferson/nsq,ploxiln/nsq,nsqio/nsq,rpau/nsq,ploxiln/nsq,bitly/nsq,mreiferson/nsq,rpau/nsq,ploxiln/nsq,ploxiln/nsq,rpau/nsq,mreiferson/nsq,nsqio/nsq,bitly/nsq,rpau/nsq,nsqio/nsq