Remove some gradle deprecation warnings (#3039)

These are trivial changes for the java/kotlin libraries.

* Specify JUnit platform explicitly
* Remove unused system property (build.test) from unit test environment
  that relied on a deprecated task property (destinationDir)
* Replace kotlinOptions.jvmTarget with compilerOptions replacement
* Update README to avoid incorrect specification of gradle 8.0, when
  other gradle 8 versions are acceptable
diff --git a/lib/kotlin/build.gradle.kts b/lib/kotlin/build.gradle.kts
index 6b1c7c9..3f8b653 100644
--- a/lib/kotlin/build.gradle.kts
+++ b/lib/kotlin/build.gradle.kts
@@ -41,7 +41,9 @@
 }
 
 tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
-    kotlinOptions.jvmTarget = "1.8"
+    compilerOptions {
+        jvmTarget = org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_1_8
+    }
 }
 
 tasks {