Mark kotlin target JVM as 1.8

There is a conflict between the compiler used for Java and Kotlin, even
if this is defined in the root project's build.gradle. Explicitly
note that the targeted jvm version is 1.8.

This was reported by Gradle and prevents the upgrade to Gradle 8.x.

api-doclet/build.gradle

@@ -6,6 +6,9 @@ description = 'Conscrypt: API Doclet'
 
 kotlin {
     jvmToolchain(11)
+    compilerOptions {
+        jvmTarget = org.jetbrains.kotlin.gradle.dsl.JvmTarget.JVM_1_8
+    }
 }
 
 dependencies {