Automated retesting: Only trigger required jobs

The job is intended to trigger required tests that flake, however it
currently triggers all tests, including optional ones that might not be
stable.

Update it to use `/retest-required` so only mandatory jobs get
triggered.

Author: alvaroaleman

config/jobs/kubernetes/sig-k8s-infra/trusted/sig-contribex-k8s-triage-robot.yaml

@@ -255,7 +255,7 @@ periodics:
   annotations:
     testgrid-dashboards: sig-contribex-k8s-triage-robot
     testgrid-tab-name: retester
-    description: Automatically /retest for approved PRs that are failing tests
+    description: Automatically /retest-required for approved PRs that are failing tests
   spec:
     containers:
     - image: gcr.io/k8s-staging-test-infra/commenter:v20250306-095fc63a16
@@ -302,7 +302,7 @@ periodics:
         - Prevent this bot from retesting with `/lgtm cancel` or `/hold`
         - Help make our tests less flaky by following our [Flaky Tests Guide][1]
 
-        /retest
+        /retest-required
 
         [1]: https://git.k8s.io/community/contributors/devel/sig-testing/flaky-tests.md
       - --template