Fix Java and Kotlin targets incompatibility in guide

Fix for the change introduced in Kotlin 1.8.0 via this issue:
https://youtrack.jetbrains.com/issue/KT-54993

Author: Tapchicoma

guide/build.gradle

@@ -5,6 +5,10 @@
 apply plugin: 'kotlin'
 apply plugin: 'kotlinx-serialization'
 
+kotlin {
+    jvmToolchain(8)
+}
+
 dependencies {
     testImplementation "org.jetbrains.kotlin:kotlin-test-junit"
     testImplementation "org.jetbrains.kotlinx:kotlinx-knit-test:$knit_version"