Try and explicitly specify Java 8 in the modules.

api/build.gradle

@@ -8,6 +8,11 @@ apply from: '../gradle/checkstyle.gradle'
 apply from: '../gradle/publish.gradle'
 apply plugin: 'com.github.johnrengelman.shadow'
 
+java {
+    sourceCompatibility = JavaVersion.VERSION_1_8
+    targetCompatibility = JavaVersion.VERSION_1_8
+}
+
 sourceSets {
     ap {
         compileClasspath += main.compileClasspath + main.output

build.gradle

@@ -43,11 +43,6 @@ allprojects {
         }
     }
 
-    java {
-        sourceCompatibility = JavaVersion.VERSION_1_8
-        targetCompatibility = JavaVersion.VERSION_1_8
-    }
-
     repositories {
         mavenLocal()
         mavenCentral()

native/build.gradle

@@ -7,6 +7,11 @@ plugins {
 apply from: '../gradle/checkstyle.gradle'
 apply from: '../gradle/publish.gradle'
 
+java {
+    sourceCompatibility = JavaVersion.VERSION_1_8
+    targetCompatibility = JavaVersion.VERSION_1_8
+}
+
 dependencies {
     implementation "com.google.guava:guava:${guavaVersion}"
     implementation "io.netty:netty-handler:${nettyVersion}"

proxy/build.gradle

@@ -8,6 +8,11 @@ plugins {
 apply from: '../gradle/checkstyle.gradle'
 apply plugin: 'com.github.johnrengelman.shadow'
 
+java {
+    sourceCompatibility = JavaVersion.VERSION_1_8
+    targetCompatibility = JavaVersion.VERSION_1_8
+}
+
 jar {
     manifest {
         def buildNumber = System.getenv("BUILD_NUMBER") ?: "unknown"