Update build variable to default to PostgreSQL 13

Author: Jonathan S. Katz

Makefile

@@ -6,7 +6,7 @@ PGO_BASEOS ?= centos8
 PGO_IMAGE_PREFIX ?= crunchydata
 PGO_IMAGE_TAG ?= $(PGO_BASEOS)-$(PGO_VERSION)
 PGO_VERSION ?= 4.6.0-beta.1
-PGO_PG_VERSION ?= 12
+PGO_PG_VERSION ?= 13
 PGO_PG_FULLVERSION ?= 13.1
 PGO_BACKREST_VERSION ?= 2.31
 PACKAGER ?= yum