Bump default pgBackRest target

This is likely a vestige but better to be bumped than not.

Author: Jonathan S. Katz

Makefile

@@ -9,7 +9,7 @@ PGO_VERSION ?= 4.7.0
 PGO_IMAGE_TAG ?= $(PGO_BASEOS)-$(PGO_VERSION)
 PGO_PG_VERSION ?= 13
 PGO_PG_FULLVERSION ?= 13.3
-PGO_BACKREST_VERSION ?= 2.31
+PGO_BACKREST_VERSION ?= 2.33
 PACKAGER ?= yum
 
 RELTMPDIR=/tmp/release.$(PGO_VERSION)