Work around openapiv2 case issue 🖍️

Building the custom task on my mac was working just fine but once we
started to build it with CI it failed with
`no required module provides package github.com/googleapis/gnostic/OpenAPIv2`

Turns out this is a known issue:
kubernetes/client-go#741

Kinda at a loss as to why other custom tasks aren't running into this
and my understanding of go mod is so shallow that I feel like I'm
writing with a crayon to fix this (nothing against crayons, they make
cool marks) - but anyway I tried to copy what the pipeline go.mod had
done since I think that's where the original version came from anyway
https://github.com/tektoncd/pipeline/blob/main/go.mod

pipeline-to-taskrun/go.mod

@@ -3,35 +3,18 @@ module github.com/tektoncd/experimental/pipeline-to-taskrun
 go 1.15
 
 require (
-	github.com/docker/go-metrics v0.0.1 // indirect
-	github.com/fsnotify/fsnotify v1.4.9 // indirect
-	github.com/fvbommel/sortorder v1.0.2 // indirect
-	github.com/gobuffalo/envy v1.7.1 // indirect
 	github.com/google/go-cmp v0.5.5
 	github.com/hashicorp/go-multierror v1.1.0
-	github.com/markbates/inflect v1.0.4 // indirect
-	github.com/rogpeppe/go-internal v1.5.2 // indirect
 	github.com/tektoncd/pipeline v0.24.0
-	github.com/theupdateframework/notary v0.7.0 // indirect
 	go.uber.org/zap v1.16.0
-	k8s.io/api v0.20.2
-	k8s.io/apimachinery v0.19.7
-	k8s.io/client-go v11.0.1-0.20190805182717-6502b5e7b1b5+incompatible
-	knative.dev/pkg v0.0.0-20210331065221-952fdd90dbb0
-	sigs.k8s.io/structured-merge-diff/v3 v3.0.1-0.20200706213357-43c19bbb7fba // indirect
+	k8s.io/api v0.20.7
+	k8s.io/apimachinery v0.20.7
+	k8s.io/client-go v0.20.7
+	knative.dev/pkg v0.0.0-20210510175900-4564797bf3b7
 )
 
 // Knative deps
 replace (
 	contrib.go.opencensus.io/exporter/stackdriver => contrib.go.opencensus.io/exporter/stackdriver v0.12.9-0.20191108183826-59d068f8d8ff
 	github.com/Azure/azure-sdk-for-go => github.com/Azure/azure-sdk-for-go v38.2.0+incompatible
 )
-
-// Pin k8s deps to v0.18.8
-replace (
-	k8s.io/api => k8s.io/api v0.18.8
-	k8s.io/apiextensions-apiserver => k8s.io/apiextensions-apiserver v0.18.8
-	k8s.io/apimachinery => k8s.io/apimachinery v0.18.8
-	k8s.io/client-go => k8s.io/client-go v0.18.8
-	k8s.io/kube-openapi => k8s.io/kube-openapi v0.0.0-20200410145947-bcb3869e6f29
-)