Remove dependency of JarContentTest on JDK 8

It turns out that `jvmTarget` and `javaHome` settings in
build.gradle.kts were changing the module settings and affected the
compilation of kotlinx-metadata-jvm sources. The correct way to use JDK
8 in tests would be to change JVM target / JDK home of the specific
KotlinCompile task via its `kotlinOptions`, but JarContentTest doesn't
need JDK 8 anyway at this moment, so simplify that instead.

libraries/kotlinx-metadata/jvm/build.gradle.kts

@@ -48,11 +48,6 @@ dependencies {
     testRuntime(project(":kotlin-reflect"))
 }
 
-tasks.named("compileTestKotlin") {
-    jvmTarget = "1.8"
-    javaHome = rootProject.extra["JDK_18"] as String
-}
-
 if (deployVersion != null) {
     publish()
 }

libraries/kotlinx-metadata/jvm/test/kotlinx/metadata/test/JarContentTest.kt

@@ -16,7 +16,6 @@ import org.junit.Assert.*
 import org.junit.Test
 import java.io.File
 import java.util.zip.ZipFile
-import kotlin.text.Charsets.UTF_8
 
 class JarContentTest {
     @Test
@@ -27,7 +26,7 @@ class JarContentTest {
     }
 
     private fun checkClassesHasNoSpecificStringConstants(jar: File) {
-        val zipFile = ZipFile(jar, UTF_8)
+        val zipFile = ZipFile(jar)
         for (entry in zipFile.entries()) {
             if (!entry.name.endsWith(".class")) continue