Compile java code to target java 8

We compile java code for the JNI wrapper which also need to target java 1.8.

Author: sstone

build.gradle.kts

@@ -23,7 +23,7 @@ buildscript {
 
 allprojects {
     group = "fr.acinq.secp256k1"
-    version = "0.17.0"
+    version = "0.17.1-SNAPSHOT"
 
     repositories {
         google()

jni/jvm/build.gradle.kts

@@ -1,3 +1,5 @@
+import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+
 plugins {
     kotlin("jvm")
     id("org.jetbrains.dokka")
@@ -31,6 +33,16 @@ dependencies {
     api(project(":jni"))
 }
 
+kotlin {
+    compilerOptions {
+        jvmTarget.set(JvmTarget.JVM_1_8)
+    }
+}
+
+java {
+    sourceCompatibility = JavaVersion.VERSION_1_8
+}
+
 publishing {
     publications {
         create<MavenPublication>("jvm") {