chore: pin plugin to v2.0.0-cf.1

[jbw976]

Description of your changes

As part of the v2.0 conformance test suite release, we should pin the plugin-crossplane.yaml file to the release version of v2.0.0-cf.1

This PR is to the release-2.0 branch, we don't want to do this in main.

I have:

• Read and followed conformance's contribution process.
• Run make reviewable test to ensure this PR is ready for review.

How has this code been tested

With this change in my local plugin-crossplane.yaml file, run:

❯ sonobuoy run --wait --plugin ./plugin-crossplane.yaml

And confirmed that the correct v2.0.0-cf.1 version of conformance tests are running:

❯ kubectl -n sonobuoy -l sonobuoy-plugin=crossplane-conformance get pod
NAME                                                   READY   STATUS    RESTARTS   AGE
sonobuoy-crossplane-conformance-job-ef40071930c74256   2/2     Running   0          2m59s

❯ kubectl -n sonobuoy -l sonobuoy-plugin=crossplane-conformance get pod -o json | jq '.items[].spec.containers[0].image'
"crossplane/conformance:v2.0.0-cf.1"