Use JVM toolchain instead of Kotlin jvmTarget

This [new Gradle feature](https://kotlinlang.org/docs/gradle-configure-project.html#gradle-java-toolchains-support) makes Gradle install the respective JDK/JRE instead of just using whatever JVM Gradle itself happens to run under. On my machine, that would be JVM 11, which leads to errors like `class file has wrong version 61.0, should be 55.0` when Dagger-generated Java files (like factories) try to interact with Kotlin-compiled classes.
The `compilerOptions.jvmTarget` is then implied.

build.gradle.kts

@@ -53,8 +53,13 @@ tasks.test {
     }
 }
 
+kotlin {
+    jvmToolchain {
+        languageVersion.set(JavaLanguageVersion.of(17))
+    }
+}
+
 tasks.withType<KotlinCompile> {
-    kotlinOptions.jvmTarget = "17"
     kotlinOptions.allWarningsAsErrors = true
     kotlinOptions.freeCompilerArgs += "-opt-in=kotlin.RequiresOptIn"
 }