Move to Go 1.16

@vdemeester told me that RH is now using 1.16 for their internal builds, which means we should be able to move off of 1.13. Hooray!

After updating `go.mod`, I ran `go mod tidy`, `go mod vendor`, and `./hack/update-codegen.sh`.

Signed-off-by: Andrew Bayer <andrew.bayer@gmail.com>

Author: abayer

go.mod

@@ -1,6 +1,6 @@
 module github.com/tektoncd/pipeline
 
-go 1.13
+go 1.16
 
 require (
 	github.com/cloudevents/sdk-go/v2 v2.5.0