Bump spilo and target version for PostgreSQL 15 (#2139)

* Bumped Spilo image tag to the one that supports PostgreSQL 15. Using CDP version temporarily until non-CDP one is released.
* Added support for PostgreSQL 15 and made it default. 9.5 and 9.6 are now no longer supported
* Bumped spilo image tag to 2.1-p9
* Bumped spilo image in test launcher

Co-authored-by: yoshihiko <ariyoshi10@gmail.com>
Co-authored-by: Felix Kunde <felix-kunde@gmx.de>

manifests/complete-postgres-manifest.yaml

@@ -10,7 +10,7 @@ metadata:
 #    "delete-date": "2020-08-31"  # can only be deleted on that day if "delete-date "key is configured
 #    "delete-clustername": "acid-test-cluster"  # can only be deleted when name matches if "delete-clustername" key is configured
 spec:
-  dockerImage: registry.opensource.zalan.do/acid/spilo-14:2.1-p7
+  dockerImage: ghcr.io/zalando/spilo-15:2.1-p9
   teamId: "acid"
   numberOfInstances: 2
   users:  # Application/Robot users
@@ -44,7 +44,7 @@ spec:
           defaultRoles: true
           defaultUsers: false
   postgresql:
-    version: "14"
+    version: "15"
     parameters:  # Expert section
       shared_buffers: "32MB"
       max_connections: "10"