Simplify build file (#300)

* Use toolchains to set JDK version for compilation

* Use standard methods to apply compiler options to compilations

* Remove API/language version override checks

These aren't necessary since kotlin-dsl is not applied to the project
anymore. There is nothing else that is setting the API or language version.

* Set JVM target version with jvmTarget

* Remove redundant suppression

build.gradle.kts

@@ -5,6 +5,7 @@ import org.jetbrains.gradle.ext.copyright
 import org.jetbrains.gradle.ext.settings
 import org.jetbrains.kotlin.gradle.dsl.KotlinVersion
 import kotlin.math.min
+import org.jetbrains.kotlin.gradle.tasks.KotlinJvmCompile
 
 plugins {
     id("org.jetbrains.kotlin.jvm") version "1.8.22"
@@ -145,7 +146,6 @@ val e2eTest: SourceSet by sourceSets.creating {
     runtimeClasspath += sourceSets["main"].output
 }
 
-@Suppress("UnstableApiUsage") // `configurations` container is incubating
 configurations {
     compatTestCompileClasspath {
         extendsFrom(testCompileClasspath.get())
@@ -170,44 +170,28 @@ sourceSets {
     }
 }
 
-kotlin.target.compilations.configureEach {
-    // Supporting Gradle 6.2+ needs to use Kotlin 1.3.
-    // See https://docs.gradle.org/current/userguide/compatibility.html
-    // For future maintainer: Kotlin 1.9.0 dropped support for Kotlin 1.3, it'll only support 1.4+.
-    // This means Gradle 7.0 will be the lowest supportable version for plugins.
-    val usedKotlinVersion = @Suppress("DEPRECATION") KotlinVersion.KOTLIN_1_3
+tasks {
+    withType<KotlinJvmCompile>().configureEach {
+        compilerOptions {
+            // Gradle fully supports running on Java 8: https://docs.gradle.org/current/userguide/compatibility.html,
+            // so we should allow users to do that too.
+            jvmTarget = JvmTarget.JVM_1_8
 
-    compilerOptions.configure {
-        // Gradle fully supports running on Java 8: https://docs.gradle.org/current/userguide/compatibility.html,
-        // so we should allow users to do that too.
-        jvmTarget = JvmTarget.fromTarget(JavaVersion.VERSION_1_8.toString())
+            // Suppress "Language version 1.3 is deprecated and its support will be removed in a future version of Kotlin".
+            freeCompilerArgs.add("-Xsuppress-version-warnings")
 
-        // Suppress "Language version 1.3 is deprecated and its support will be removed in a future version of Kotlin".
-        freeCompilerArgs.add("-Xsuppress-version-warnings")
-    }
-    compileTaskProvider.configure {
-        // These two (api & lang) needs to be here instead of in compilations.compilerOptions.configure { },
-        // to prevent KotlinDslCompilerPlugins overriding to Kotlin 1.8.
-        compilerOptions.apiVersion = usedKotlinVersion
-        // Theoretically we could use newer language version here,
-        // but sadly the @kotlin.Metadata created on the classes would be incompatible with older consumers.
-        compilerOptions.languageVersion = usedKotlinVersion
+            // Supporting Gradle 6.2+ needs to use Kotlin 1.3.
+            // See https://docs.gradle.org/current/userguide/compatibility.html
+            // For future maintainer: Kotlin 1.9.0 dropped support for Kotlin 1.3, it'll only support 1.4+.
+            // This means Gradle 7.0 will be the lowest supportable version for plugins.
+            val usedKotlinVersion = @Suppress("DEPRECATION") KotlinVersion.KOTLIN_1_3
 
-        // Validate that we're using the right version.
-        doFirst {
-            val api = compilerOptions.apiVersion.get()
-            val language = compilerOptions.languageVersion.get()
-            if (api != usedKotlinVersion || language != usedKotlinVersion) {
-                TODO(
-                    "There's mismatch between configured and actual versions:\n" +
-                            "apiVersion=${api}, languageVersion=${language}, configured=${usedKotlinVersion}."
-                )
-            }
+            apiVersion = usedKotlinVersion
+            // Theoretically we could use newer language version here,
+            // but sadly the @kotlin.Metadata created on the classes would be incompatible with older consumers.
+            languageVersion = usedKotlinVersion
         }
     }
-}
-
-tasks {
     shadowJar {
         exclude("META-INF/maven/**", "META-INF/proguard/**", "META-INF/*.kotlin_module")
         manifest {