Update to Kotlin 1.5 (and associated deps) (#1339)

adapters/build.gradle.kts

@@ -14,12 +14,20 @@
  * limitations under the License.
  */
 
+import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
+
 plugins {
   kotlin("jvm")
   id("com.vanniktech.maven.publish")
   id("ru.vyarus.animalsniffer")
 }
 
+tasks.withType<KotlinCompile>().configureEach {
+  kotlinOptions {
+    jvmTarget = "1.6"
+  }
+}
+
 dependencies {
   compileOnly(Dependencies.jsr305)
   api(project(":moshi"))