fix(schemas): remove installplan generation field requirement

Fix incompatible schema change by making new(ish) InstallPlan generation
field optional.

(upstream api commit: 6f6bae0b60abdac3b603d7c08dfbeb512c1a382f)

Author: njhale

staging/api/crds/operators.coreos.com_installplans.yaml

@@ -59,7 +59,6 @@ spec:
             - approval
             - approved
             - clusterServiceVersionNames
-            - generation
             properties:
               approval:
                 description: Approval is the user approval policy for an InstallPlan.