Removes PG15 (#178)

Removes PG15 images since we are dropping PG15 support in Citus.

Sister PR citusdata/citus#8372

Author: naisila

circleci/images/Makefile

@@ -18,9 +18,7 @@ STYLE_CHECKER_TOOLS_VERSION=0.8.33
 
 # Upgrade tests for the PG major versions from PG_VERSIONS file 
 CITUS_UPGRADE_PG_VERSIONS=$(shell head PG_VERSIONS|cut -c 6-|tr '\n' ' ')
-# 11.1.0 is the oldest version supporting PG15, whereas 11.3.0 is for upgrade_pg_dist_cleanup tests 
 # 12.1.10 is the latest release of Citus 12 when the test is expanded to cover Citus 12
-CITUS_UPGRADE_VERSIONS_15=v11.1.0 v11.3.0 v12.1.10
 CITUS_UPGRADE_VERSIONS_16=v12.1.10
 # Latest minor version of Citus 13
 CITUS_UPGRADE_VERSIONS_17=v13.2.0

circleci/images/PG_VERSIONS

@@ -1,3 +1,2 @@
-PG15=15.14
 PG16=16.10
 PG17=17.6