Update gitea-credentials ExternalSecret Vault path

Change key from 'gitea' to 'platform/gitea' to match the
new categorized Vault secret structure.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -14,9 +14,9 @@ spec:
   data:
     - secretKey: username
       remoteRef:
-        key: gitea
+        key: platform/gitea
         property: username
     - secretKey: password
       remoteRef:
-        key: gitea
+        key: platform/gitea
         property: password