Add CITUS_UPGRADE_VERSIONS_18 for PG18 exttester image

[eaydingol]

Description

Add CITUS_UPGRADE_VERSIONS_18=v14.0.0 to the Makefile so the exttester image for PG18 includes pre-built Citus 14.0 binaries.

Check

$ docker run -it ghcr.io/citusdata/exttester:18.1-dev-181ce3c bash
root@2a07cb905f71:/home/circleci# ls /opt/citus-versions/
v14.0.0
root@2a07cb905f71:/home/circleci# ls /opt/citus-versions/v14.0.0/
citus.so  citus_columnar.so  citus_decoders  citus_pgoutput.so  citus_wal2json.so

[ihalatci]

do we still need the "-n2" here?

[eaydingol]

we don't, we can add citus upgrade tests for pg 18 (citus 14 to citus 15-devel)
removed it in the last commit.

[ihalatci]

closign in favor of #194 where we include the fix for style checker build issue