Build: Make ProGuard task cacheable

Exclude jdk files form libraries input. Instead add jdk major version
to inputs. JavaCompile task acts same to ignore fluctuations in JDK
implementations since api should remain same

build.gradle.kts

@@ -30,7 +30,6 @@ buildscript {
         classpath("org.jetbrains.kotlin:kotlin-build-gradle-plugin:0.0.15")
         classpath("com.gradle.publish:plugin-publish-plugin:0.9.7")
         classpath(kotlin("gradle-plugin", bootstrapKotlinVersion))
-        classpath("net.sf.proguard:proguard-gradle:6.1.0")
         classpath("org.jetbrains.dokka:dokka-gradle-plugin:0.9.17")
     }
 }