set jvmTarget back to 8 (#258)

To fix #257.
* Update build.gradle.kts

Author: oshai

build.gradle.kts

@@ -38,7 +38,7 @@ kotlin {
             kotlinOptions {
                 apiVersion = "1.4"
                 languageVersion = "1.4"
-                jvmTarget = "11"
+                jvmTarget = "1.8"
             }
         }
     }