Raise the minimum supported version of Kotlin to 1.6

Closes gh-29660

Author: snicoll

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

@@ -30,7 +30,7 @@
  * <ul>
  * <li>{@link KotlinCompile} tasks are configured to:
  * <ul>
- * <li>Use {@code apiVersion} and {@code languageVersion} 1.3.
+ * <li>Use {@code apiVersion} and {@code languageVersion} 1.6.
  * <li>Use {@code jvmTarget} 1.8.
  * <li>Treat all warnings as errors
  * <li>Suppress version warnings
@@ -50,8 +50,8 @@ void apply(Project project) {
 
 	private void configure(KotlinCompile compile) {
 		KotlinJvmOptions kotlinOptions = compile.getKotlinOptions();
-		kotlinOptions.setApiVersion("1.3");
-		kotlinOptions.setLanguageVersion("1.3");
+		kotlinOptions.setApiVersion("1.6");
+		kotlinOptions.setLanguageVersion("1.6");
 		kotlinOptions.setJvmTarget("1.8");
 		kotlinOptions.setAllWarningsAsErrors(true);
 		List<String> freeCompilerArgs = new ArrayList<>(compile.getKotlinOptions().getFreeCompilerArgs());