CI: update Tekton pipelines

- Pull pipelines use odh-pr tag
- Push pipelines use odh-stable tag
- Target branch: main

Author: github-actions[bot]

.tekton/trainer-pull-request.yaml

@@ -10,7 +10,7 @@ metadata:
     pipelinesascode.tekton.dev/cancel-in-progress: "false"
     pipelinesascode.tekton.dev/max-keep-runs: "3"
     pipelinesascode.tekton.dev/on-cel-expression: event == "pull_request" && target_branch
-      == "main"
+      == "stable"
   creationTimestamp: null
   labels:
     appstudio.openshift.io/application: opendatahub-builds

.tekton/trainer-push.yaml

@@ -8,7 +8,7 @@ metadata:
     pipelinesascode.tekton.dev/cancel-in-progress: "false"
     pipelinesascode.tekton.dev/max-keep-runs: "3"
     pipelinesascode.tekton.dev/on-cel-expression: event == "push" && target_branch
-      == "main"
+      == "stable"
   creationTimestamp: null
   labels:
     appstudio.openshift.io/application: opendatahub-builds