fix kotlin cross test by downgrading to java 8 (#2840)

Author: jimexist

lib/kotlin/cross-test-client/build.gradle.kts

@@ -52,7 +52,7 @@ dependencies {
 
 kotlin {
     jvmToolchain {
-        (this as JavaToolchainSpec).languageVersion.set(JavaLanguageVersion.of(11))
+        (this as JavaToolchainSpec).languageVersion.set(JavaLanguageVersion.of(8))
     }
 }
 

lib/kotlin/cross-test-server/build.gradle.kts

@@ -50,7 +50,7 @@ dependencies {
 
 kotlin {
     jvmToolchain {
-        (this as JavaToolchainSpec).languageVersion.set(JavaLanguageVersion.of(11))
+        (this as JavaToolchainSpec).languageVersion.set(JavaLanguageVersion.of(8))
     }
 }