Add replacement for cluster resource in "v1alpha1 to v1beta1 migration" docs
Add replacement for cluster resource in "v1alpha1 to v1beta1 migration" docs

This is part of providing documentation for alternative to pipeline resource.
As we have now kubeconfig-creator task in the catalog as the replacement for cluster resource,
Added the same in the "v1alpha1 to v1beta1 migration" docs.
Ref: #1369

Signed-off-by: Divyansh42 <900fa4a0dcc22124fa2a3b2378fe93929a62b07e@redhat.com>

tektoncd/pipeline,tektoncd/pipeline