Align JVM target version for Kotlin and Java compile tasks

This commit resolves a build error related to mismatched JVM target
versions for the Kotlin and Java compile tasks. Previously, the
'compileJava' task was targeting JVM 11, while the 'compileKotlin'
task was targeting JVM 8.

Both tasks have now been set to target JVM 11, ensuring consistency
and eliminating the build error.

bdk-jvm/lib/build.gradle.kts

@@ -20,8 +20,8 @@ repositories {
 }
 
 java {
-    sourceCompatibility = JavaVersion.VERSION_1_8
-    targetCompatibility = JavaVersion.VERSION_1_8
+    sourceCompatibility = JavaVersion.VERSION_11
+    targetCompatibility = JavaVersion.VERSION_11
     withSourcesJar()
     withJavadocJar()
 }
@@ -107,4 +107,8 @@ signing {
 // binaries before running the tests
 tasks.withType<KotlinCompile> {
     dependsOn("buildJvmLib")
+
+    kotlinOptions {
+        jvmTarget = "11"
+    }
 }