prep kubernetes/org jobs for default branch rename

specificalky
- add main to any pre/postsubmit triggers that reference master
- try removing explicit master extra_ref, see if that gets us default branch

config/jobs/kubernetes/test-infra/test-infra-trusted.yaml

@@ -4,6 +4,7 @@ postsubmits:
     cluster: test-infra-trusted
     decorate: true
     branches:
+    - ^main$
     - ^master$
     max_concurrency: 1
     spec:
@@ -784,7 +785,6 @@ periodics:
   extra_refs:
   - org: kubernetes
     repo: org
-    base_ref: master
   spec:
     containers:
     - image: launcher.gcr.io/google/bazel:0.29.1