Remove Tekton pipeline configuration files for the main branch from the release-4.20 branch. These pipelines are not needed on the release branch as they are configured to only trigger on the main branch.

Files being removed:

• .tekton/control-plane-operator-main-pull-request.yaml
• .tekton/control-plane-operator-main-push.yaml
• .tekton/hypershift-operator-main-pull-request.yaml
• .tekton/hypershift-operator-main-push.yaml
• .tekton/hypershift-operator-main-tag.yaml
• .tekton/hypershift-shared-ingress-main-pull-request.yaml
• .tekton/hypershift-shared-ingress-main-push.yaml

All of these pipeline configurations have target_branch == "main" conditions in their CEL expressions, making them irrelevant on the release-4.20 branch.