Configure Kotlin compilation JVM target by convention

Closes gh-29952

buildSrc/src/main/java/org/springframework/boot/build/KotlinConventions.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2021 the original author or authors.
+ * Copyright 2012-2022 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -31,6 +31,7 @@ import org.jetbrains.kotlin.gradle.tasks.KotlinCompile;
  * <li>{@link KotlinCompile} tasks are configured to:
  * <ul>
  * <li>Use {@code apiVersion} and {@code languageVersion} 1.3.
+ * <li>Use {@code jvmTarget} 1.8.
  * <li>Treat all warnings as errors
  * <li>Suppress version warnings
  * </ul>
@@ -48,6 +49,7 @@ class KotlinConventions {
 				KotlinJvmOptions kotlinOptions = compile.getKotlinOptions();
 				kotlinOptions.setApiVersion("1.3");
 				kotlinOptions.setLanguageVersion("1.3");
+				kotlinOptions.setJvmTarget("1.8");
 				kotlinOptions.setAllWarningsAsErrors(true);
 				List<String> freeCompilerArgs = new ArrayList<>(compile.getKotlinOptions().getFreeCompilerArgs());
 				freeCompilerArgs.add("-Xsuppress-version-warnings");

spring-boot-project/spring-boot/build.gradle

@@ -161,18 +161,6 @@ sourceSets {
 	}
 }
 
-compileKotlin {
-	kotlinOptions {
-		jvmTarget = 1.8
-	}
-}
-
-compileTestKotlin {
-	kotlinOptions {
-		jvmTarget = 1.8
-	}
-}
-
 toolchain {
 	testJvmArgs.add("--add-opens=java.base/java.net=ALL-UNNAMED")
 }

spring-boot-tests/spring-boot-smoke-tests/spring-boot-smoke-test-webflux-coroutines/build.gradle

@@ -6,18 +6,6 @@ plugins {
 
 description = "Spring Boot WebFlux coroutines smoke test"
 
-compileKotlin {
-	kotlinOptions {
-		jvmTarget = "1.8"
-	}
-}
-
-compileTestKotlin {
-	kotlinOptions {
-		jvmTarget = "1.8"
-	}
-}
-
 dependencies {
 	implementation(project(":spring-boot-project:spring-boot-starters:spring-boot-starter-webflux"))
 	implementation("com.fasterxml.jackson.module:jackson-module-kotlin")