build: Pin CI Neo4j version to 5.20.

Author: michael-simons

ci/test.sh

@@ -9,6 +9,7 @@ export DEVELOCITY_CACHE_USERNAME=${DEVELOCITY_CACHE_USR}
 export DEVELOCITY_CACHE_PASSWORD=${DEVELOCITY_CACHE_PSW}
 export JENKINS_USER=${JENKINS_USER_NAME}
 export SDN_FORCE_REUSE_OF_CONTAINERS=true
+export SDN_NEO4J_VERSION=5.20
 
 # The environment variable to configure access key is still GRADLE_ENTERPRISE_ACCESS_KEY
 export GRADLE_ENTERPRISE_ACCESS_KEY=${DEVELOCITY_ACCESS_KEY}