Pin ubuntu-jammy for openapi-identity job

Keystone implicitly requires pillow library which is not having wheels
on ubuntu-noble. In this repo itself this issue is fixed by adding few
libraries into the bindep, but this is not possible when the job is used
in the keystone repo directly.

Change-Id: I59ccfe9fd4e27ce3d5d27f3eb5b179ca16d0a542

zuul.d/openapi.yaml

@@ -96,6 +96,7 @@
     parent: codegenerator-openapi-tips-base
     description: |
       Generate OpenAPI spec for Keystone
+    nodeset: ubuntu-jammy
     required-projects:
       - name: openstack/keystone