FIX(tekton): correct github-credentials secret keys

- Change PAT to username and password properties
- Match Vault secret structure used by argocd

tekton/ci-cd/manifests/secrets/github-credentials.yaml

@@ -12,7 +12,11 @@ spec:
     name: github-credentials
     creationPolicy: Owner
   data:
-    - secretKey: token
+    - secretKey: username
       remoteRef:
         key: github
-        property: PAT
+        property: username
+    - secretKey: password
+      remoteRef:
+        key: github
+        property: password