Update to Kotlin 1.60 and Gradle 4.9

Removes the dependency on legacy script compilation and now relies on script
discovery.  In addition, the Gradle build scripts were refactored and updated
to be compatible with Gradle 5.0 and make use of the new java-library
configurations.

cache/build.gradle

@@ -1,5 +1,10 @@
+plugins {
+    id "java-library"
+}
+
 description = 'Apollo Cache'
 
 dependencies {
-    compile project(':util')
+    implementation project(':util')
+    implementation group: 'com.google.guava', name: 'guava', version: guavaVersion
 }