build(gradle-plugin): Prepare for upgrading to Gradle 9

Gradle 9 will raise the built-in Kotlin version [1], so ensure
compatibility with older Gradle versions using older Kotlin versions.

[1]: https://docs.gradle.org/9.0.0-milestone-9/userguide/upgrading_version_8.html#plugins_written_with_the_kotlin_dsl_require_gradle_8_11_by_default

Signed-off-by: Sebastian Schuberth <[email protected]>

Author: sschuberth

plugins/package-managers/gradle-plugin/build.gradle.kts

@@ -18,6 +18,7 @@
  */
 
 import org.jetbrains.kotlin.gradle.dsl.JvmTarget
+import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
 import org.jetbrains.kotlin.gradle.tasks.KotlinCompile
 
 plugins {
@@ -54,6 +55,11 @@ tasks.named<JavaCompile>("compileJava") {
 tasks.named<KotlinCompile>("compileKotlin") {
     compilerOptions {
         jvmTarget = gradleToolingApiLowestSupportedJavaVersion
+
+        // See https://docs.gradle.org/current/userguide/compatibility.html#kotlin.
+        freeCompilerArgs = listOf("-Xsuppress-version-warnings")
+        languageVersion = @Suppress("DEPRECATION") KotlinVersion.KOTLIN_1_7
+        apiVersion = @Suppress("DEPRECATION") KotlinVersion.KOTLIN_1_7
     }
 }