solution: use Java 13 as minimum supported for compilation

build.gradle

@@ -23,7 +23,7 @@ plugins {
 
     id 'org.jetbrains.kotlin.jvm' version '1.3.70'
     id 'com.google.cloud.tools.jib' version "2.5.0"
-    id 'org.springframework.boot' version '2.1.4.RELEASE'
+    id 'org.springframework.boot' version '2.3.5.RELEASE'
     id 'io.spring.dependency-management' version '1.0.6.RELEASE'
     id 'com.palantir.git-version' version '0.12.2'
     id "com.google.protobuf" version "0.8.12"
@@ -38,8 +38,8 @@ group = 'io.emeraldpay.dshackle'
 version = '0.10.0-SNAPSHOT'
 
 java {
-    sourceCompatibility = JavaVersion.VERSION_11
-    targetCompatibility = JavaVersion.VERSION_11
+    sourceCompatibility = JavaVersion.VERSION_13
+    targetCompatibility = JavaVersion.VERSION_13
 }
 
 repositories {
@@ -123,16 +123,17 @@ dependencies {
     testImplementation 'org.objenesis:objenesis:3.1'
     testImplementation 'org.mock-server:mockserver-netty:5.10'
     testImplementation "nl.jqno.equalsverifier:equalsverifier:3.3"
+    testImplementation "org.codehaus.groovy:groovy:${groovyVersion}"
 }
 
 compileKotlin {
     kotlinOptions {
-        jvmTarget = "11"
+        jvmTarget = "13"
     }
 }
 compileTestKotlin {
     kotlinOptions {
-        jvmTarget = "11"
+        jvmTarget = "13"
     }
 }