Raise the minimum supported version of Kotlin to 1.7

Closes gh-31391

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

@@ -30,7 +30,7 @@ import org.jetbrains.kotlin.gradle.tasks.KotlinCompile;
  * <ul>
  * <li>{@link KotlinCompile} tasks are configured to:
  * <ul>
- * <li>Use {@code apiVersion} and {@code languageVersion} 1.6.
+ * <li>Use {@code apiVersion} and {@code languageVersion} 1.7.
  * <li>Use {@code jvmTarget} 17.
  * <li>Treat all warnings as errors
  * <li>Suppress version warnings
@@ -50,8 +50,8 @@ class KotlinConventions {
 
 	private void configure(KotlinCompile compile) {
 		KotlinJvmOptions kotlinOptions = compile.getKotlinOptions();
-		kotlinOptions.setApiVersion("1.6");
+		kotlinOptions.setApiVersion("1.7");
-		kotlinOptions.setLanguageVersion("1.6");
+		kotlinOptions.setLanguageVersion("1.7");
 		kotlinOptions.setJvmTarget("17");
 		kotlinOptions.setAllWarningsAsErrors(true);
 		List<String> freeCompilerArgs = new ArrayList<>(compile.getKotlinOptions().getFreeCompilerArgs());

spring-boot-project/spring-boot-docs/src/docs/asciidoc/features/kotlin.adoc

@@ -13,7 +13,7 @@ Feel free to join the #spring channel of https://slack.kotlinlang.org/[Kotlin Sl
 
 [[features.kotlin.requirements]]
 === Requirements
-Spring Boot requires at least Kotlin 1.6.x and manages a suitable Kotlin version through dependency management.
+Spring Boot requires at least Kotlin 1.7.x and manages a suitable Kotlin version through dependency management.
 To use Kotlin, `org.jetbrains.kotlin:kotlin-stdlib` and `org.jetbrains.kotlin:kotlin-reflect` must be present on the classpath.
 The `kotlin-stdlib` variants `kotlin-stdlib-jdk7` and `kotlin-stdlib-jdk8` can also be used.