build: set kotlin compiler targets for user compatibility

This re-introduces compatibility for consuming users dependant on earlier
versions of the Kotlin compiler, while still allowing usage of latest
Kotlin toolchains within the project itself.

We've set the minimum supported version to 1.9.0 as of this commit,
which should revert the breaking change introduced with version
[2.1.1](https://github.com/navikt/mock-oauth2-server/releases/tag/2.1.11),
where the compiler minor version was incremented.

Huge thanks to this massively useful blog post:
https://blog.alllex.me/posts/2023-11-03-liberal-library-tooling/

Author: tronghn

build.gradle.kts

@@ -1,6 +1,7 @@
 import java.time.Duration
 import com.github.benmanes.gradle.versions.updates.DependencyUpdatesTask
 import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
 
 val assertjVersion = "3.27.3"
 val kotlinLoggingVersion = "3.0.5"
@@ -10,7 +11,6 @@ val mockWebServerVersion = "4.12.0"
 val jacksonVersion = "2.19.0"
 val nettyVersion = "4.2.2.Final"
 val junitJupiterVersion = "5.13.0"
-val kotlinVersion = "2.1.21"
 val freemarkerVersion = "2.3.34"
 val kotestVersion = "5.9.1"
 val bouncyCastleVersion = "1.81"
@@ -24,7 +24,7 @@ val mainClassKt = "no.nav.security.mock.oauth2.StandaloneMockOAuth2ServerKt"
 
 plugins {
     application
-    kotlin("jvm") version "2.1.21"
+    alias(libs.plugins.kotlin.jvm) // refers to plugin declared in gradle/libs.versions.toml
     id("se.patrikerdes.use-latest-versions") version "0.2.18"
     id("com.github.ben-manes.versions") version "0.52.0"
     id("org.jmailen.kotlinter") version "5.1.0"
@@ -49,6 +49,28 @@ java {
     withSourcesJar()
 }
 
+kotlin {
+    val kotlinTarget = libs.versions.kotlinTarget
+    val kotlinTargetVersion = kotlinTarget.map {
+        KotlinVersion.fromVersion(it.toKotlinMinor())
+    }
+
+    compilerOptions {
+        languageVersion = kotlinTargetVersion
+        apiVersion = kotlinTargetVersion
+        // Syncing Kotlin JVM target with Java plugin JVM target
+        jvmTarget = JvmTarget.JVM_17
+    }
+
+    // Setting core libraries version to manage compile and runtime dependencies exposed in the published artifact metadata
+    // These will become transitive dependencies for our users.
+    // Core libraries for JVM are kotlin-stdlib and kotlin-test.
+    coreLibrariesVersion = kotlinTarget.get()
+}
+
+// 1.7.21 => 1.7, 1.9 => 1.9
+fun String.toKotlinMinor() = split(".").take(2).joinToString(".")
+
 apply(plugin = "org.jmailen.kotlinter")
 
 repositories {
@@ -73,7 +95,7 @@ dependencies {
     testImplementation("org.junit.jupiter:junit-jupiter-params:$junitJupiterVersion")
     testImplementation("io.kotest:kotest-runner-junit5-jvm:$kotestVersion") // for kotest framework
     testImplementation("io.kotest:kotest-assertions-core-jvm:$kotestVersion") // for kotest core jvm assertions
-    testImplementation("org.jetbrains.kotlin:kotlin-test-junit5:$kotlinVersion")
+    testImplementation("org.jetbrains.kotlin:kotlin-test-junit5") // uses version matching kotlin-jvm plugin
     testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine:$junitJupiterVersion")
     // example use with different frameworks
     testImplementation("org.springframework.boot:spring-boot-starter-webflux:$springBootVersion")
@@ -289,12 +311,6 @@ tasks {
         }
     }
 
-    withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
-        compilerOptions {
-            jvmTarget.set(JvmTarget.JVM_17)
-        }
-    }
-
     withType<Test> {
         jvmArgs("--add-opens=java.base/java.util=ALL-UNNAMED")
         useJUnitPlatform()

gradle/libs.versions.toml

@@ -0,0 +1,6 @@
+[versions]
+kotlinTarget = "1.9.0" # Minimum supported Kotlin version for consumers of the library
+kotlinToolchain = "2.1.21" # Actual version used by tooling within this project
+
+[plugins]
+kotlin-jvm = { id = "org.jetbrains.kotlin.jvm", version.ref = "kotlinToolchain" }