Externalise dependency versions and update junit and caffeine to latest versions.

Author: Harrison Cole

gradle.properties

@@ -15,4 +15,13 @@ org.gradle.caching=true
 
 # Bespoke settings.
 projectTitle = Java Dataloader
-projectDescription = Port of Facebook Dataloader for Java
+projectDescription = Port of Facebook Dataloader for Java
+
+# Dependency versions.
+junit_version=5.11.3
+hamcrest_version=2.2
+slf4j_version=1.7.30
+awaitility_version=2.0.0
+reactor_core_version=3.6.6
+caffeine_version=3.1.8
+reactive_streams_version=1.0.3