Expected behavior

It's only possible to prune pipeline runs and task runs

Actual behavior

Generated cron job tries to delete whatever resource type that user puts into Tekton config which will fail when cron job runs.

Steps to reproduce

1. Install Pipelines 1.6
2. Add the following configuration to tekton config (oc edit tektonconfig config)
3. Verify the content of cron job in openshift-pipelines namespace

pruner:
    keep: 20
    resources:
    - pipelinerun
    - taskrun
    - foo
    - bar
    schedule: '* * * * *'

Cron job contains similar commands:

args:
        - >-
          tkn pipelinerun delete --keep=20 -n=catalog1 -f ; tkn taskrun
          delete --keep=20 -n=catalog1 -f ; tkn foo delete --keep=20
          -n=catalog1 -f ; tkn bar delete --keep=20 -n=catalog1 -f ;