Upgrade Gradle Shadow plugin to 2.0.2

With this commit we upgrade the Gradle Shadow plugin that is used in our
benchmarks to version 2.0.2. This version does not use APIs that are
deprecated in Gradle 4.x.

benchmarks/build.gradle

@@ -24,7 +24,7 @@ buildscript {
         }
     }
     dependencies {
-        classpath 'com.github.jengelman.gradle.plugins:shadow:1.2.4'
+        classpath 'com.github.jengelman.gradle.plugins:shadow:2.0.2'
     }
 }
 

client/benchmark/build.gradle

@@ -24,7 +24,7 @@ buildscript {
     }
   }
   dependencies {
-    classpath 'com.github.jengelman.gradle.plugins:shadow:1.2.3'
+    classpath 'com.github.jengelman.gradle.plugins:shadow:2.0.2'
   }
 }