Require At Lead One Repo to be Configured

At least one pgBackRest repository must be defined within a
PostgresCluster spec for the proper operation of any/all
PostgreSQL clusters created by PGO.

This commit therefore updates the validation for the "repos"
section of the spec to require that at least one repo always
be defined when creating or modifying a PostgresCluster.

Issue: [ch12418]

Author: andrewlecuyer

config/crd/bases/postgres-operator.crunchydata.com_postgresclusters.yaml

@@ -1422,6 +1422,7 @@ spec:
                           required:
                           - name
                           type: object
+                        minItems: 1
                         type: array
                         x-kubernetes-list-map-keys:
                         - name
@@ -2219,6 +2220,8 @@ spec:
                         - enabled
                         - repoName
                         type: object
+                    required:
+                    - repos
                     type: object
                 required:
                 - pgbackrest