head-node: Upgrade to PostgreSQL 15.

Data has been migrated via ‘pg_dumpall’ as explained at
<https://www.postgresql.org/docs/current/upgrading.html>.

* head-node.scm (postgreslq-configuration)[postgreslq]: Change to
POSTGRESLQ-15.

head-node.scm

@@ -510,7 +510,11 @@ export CUIRASS_LOGGING_LEVEL=${CUIRASS_LOGGING_LEVEL:-debug}\n"
                  ;; <https://www.postgresql.org/docs/current/runtime-config-connection.html>.
                  (service postgresql-service-type
                           (postgresql-configuration
-                           (postgresql postgresql-10)
+                           ;; See
+                           ;; <https://www.postgresql.org/docs/current/upgrading.html>
+                           ;; on how to upgrade the database when migrating
+                           ;; to a newer version of PostgreSQL.
+                           (postgresql postgresql-15)
                            (config-file
                             (postgresql-config-file
                              (extra-config