Fixed NoSuchMethodError when parsing a JSON stream on Java 8

Fixes #2326

In Java 8 and earlier, the `position(I)` method was final in `Buffer`, and returned a `Buffer`.
In Java 9 and later, the method was opened, and `ByteBuffer` overrides it, returning a `ByteBuffer`.
This causes a `NoSuchMethodError` when running a class, compiled with a newer Java version, on Java 8.

Adding `-Xjdk-release=1.8` parameter allows the compiler to generate bytecode compatible with JVM 1.8

build.gradle

@@ -151,6 +151,12 @@ def excludedFromBomProjects = unpublishedProjects + "kotlinx-serialization-bom"
 def uncoveredProjects = ["kotlinx-serialization-bom", "benchmark", "guide"] as Set
 
 subprojects {
+    tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach { task ->
+        if (!(task.name.contains("Test") || task.name.contains("Jmh"))) {
+            task.kotlinOptions.freeCompilerArgs += "-Xjdk-release=1.8"
+        }
+    }
+
     tasks.withType(org.jetbrains.kotlin.gradle.tasks.AbstractKotlinCompile).all { task ->
         if (task.name.contains("Test") || task.name.contains("Jmh")) {
             task.kotlinOptions.freeCompilerArgs += experimentalsInTestEnabled

formats/json/jvmMain/src/kotlinx/serialization/json/internal/CharsetReader.kt

@@ -20,20 +20,9 @@ internal class CharsetReader(
             .onMalformedInput(CodingErrorAction.REPLACE)
             .onUnmappableCharacter(CodingErrorAction.REPLACE)
         byteBuffer = ByteBuffer.wrap(ByteArrayPool8k.take())
-        // An explicit cast is needed here due to an API change in Java 9, see #2218.
-        //
-        // In Java 8 and earlier, the `flip` method was final in `Buffer`, and returned a `Buffer`.
-        // In Java 9 and later, the method was opened, and `ByteFuffer` overrides it, returning a `ByteBuffer`.
-        //
-        // You could observe this by decompiling this call with `javap`:
-        // Compiled with Java 8 it produces `INVOKEVIRTUAL java/nio/ByteBuffer.flip ()Ljava/nio/Buffer;`
-        // Compiled with Java 9+ it produces `INVOKEVIRTUAL java/nio/ByteBuffer.flip ()Ljava/nio/ByteBuffer;`
-        //
-        // This causes a `NoSuchMethodError` when running a class, compiled with a newer Java version, on Java 8.
-        //
         // To mitigate that, `--bootclasspath` / `--release` options were introduced in `javac`, but there are no
         // counterparts for these options in `kotlinc`, so an explicit cast is required.
-        (byteBuffer as Buffer).flip() // Make empty
+        byteBuffer.flip() // Make empty
     }
 
     @Suppress("NAME_SHADOWING")
@@ -105,7 +94,7 @@ internal class CharsetReader(
             if (bytesRead < 0) return bytesRead
             byteBuffer.position(position + bytesRead)
         } finally {
-            (byteBuffer as Buffer).flip() // see the `init` block in this class for the reasoning behind the cast
+            byteBuffer.flip() // see the `init` block in this class for the reasoning behind the cast
         }
         return byteBuffer.remaining()
     }