Upgrade Flink benchmarks to Flink 1.13

Fix #302 (closed). Depends on !223 (merged).

theodolite-benchmarks/buildSrc/src/main/groovy/theodolite.flink.gradle

@@ -20,7 +20,7 @@ shadowJar {
 tasks.distZip.enabled = false
 
 ext {
-  flinkVersion = '1.12.2'
+  flinkVersion = '1.13.5'
   scalaBinaryVersion = '2.12'
 }