tooling: Remove ubuntu-18.04 from default builds

Author: cassava

Makefile.docker

@@ -6,7 +6,7 @@ PROJECT_ROOT := $(dir $(abspath $(lastword ${MAKEFILE_LIST})))
 PROJECT_VERSION := $(shell make --no-print-directory -C ${PROJECT_ROOT} -f Makefile.package info-version)
 
 UBUNTU_NAME := ubuntu
-UBUNTU_VERSIONS := 18.04 20.04 22.04
+UBUNTU_VERSIONS := 20.04 22.04
 
 DOCKER := DOCKER_BUILDKIT=1 docker
 DOCKER_IMAGE_NAME := cloe/cloe-engine