Bump Gradle Shadow version

We'll need to upgrade to Gradle 7 soon, probably also with an upgrade to Kotlin DSL too...

build.gradle

@@ -1,9 +1,6 @@
 buildscript {
-    repositories {
-        jcenter()
-    }
     dependencies {
-        classpath 'com.github.jengelman.gradle.plugins:shadow:5.2.0'
+        classpath 'com.github.jengelman.gradle.plugins:shadow:6.1.0'
     }
 }