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/java/gradle/environment.gradle b/lib/java/gradle/environment.gradle
index 977c07e..7e05b87 100644
--- a/lib/java/gradle/environment.gradle
+++ b/lib/java/gradle/environment.gradle
@@ -69,5 +69,6 @@
 
     testImplementation "org.junit.jupiter:junit-jupiter:${junitVersion}"
     testImplementation "org.mockito:mockito-core:${mockitoVersion}"
+    testRuntimeOnly "org.junit.platform:junit-platform-launcher"
     testRuntimeOnly "org.slf4j:slf4j-log4j12:${slf4jVersion}"
 }