misc: add -Xjdk-release=1.8 everywhere we set jvmTarget

.changes/91619ed5-0056-4f22-8863-8dd35b7a3c21.json

@@ -0,0 +1,8 @@
+{
+    "id": "91619ed5-0056-4f22-8863-8dd35b7a3c21",
+    "type": "bugfix",
+    "description": "Enable building this project on JDK21 by setting -Xjdk-release flag",
+    "issues": [
+        "https://github.com/smithy-lang/smithy-kotlin/issues/1295"
+    ]
+}

aws-runtime/build.gradle.kts

@@ -63,6 +63,7 @@ subprojects {
     tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
         compilerOptions {
             jvmTarget.set(JvmTarget.JVM_1_8)
+            freeCompilerArgs.add("-Xjdk-release=1.8")
             freeCompilerArgs.add("-Xexpect-actual-classes")
         }
     }

codegen/aws-sdk-codegen/build.gradle.kts

@@ -60,6 +60,7 @@ val generateSdkRuntimeVersion by tasks.registering {
 tasks.withType<KotlinCompile> {
     compilerOptions {
         jvmTarget.set(JvmTarget.JVM_17)
+        freeCompilerArgs.add("-Xjdk-release=17")
     }
     dependsOn(generateSdkRuntimeVersion)
 }

dokka-aws/build.gradle.kts

@@ -35,6 +35,7 @@ tasks.withType<Test>().configureEach {
 tasks.withType<KotlinCompile> {
     compilerOptions {
         jvmTarget.set(JvmTarget.JVM_1_8)
+        freeCompilerArgs.add("-Xjdk-release=1.8")
         allWarningsAsErrors.set(false) // FIXME Dokka bundles stdlib into the classpath, causing an unfixable warning
     }
 }

hll/build.gradle.kts

@@ -86,6 +86,7 @@ subprojects {
     tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
         compilerOptions {
             jvmTarget.set(JvmTarget.JVM_1_8)
+            freeCompilerArgs.add("-Xjdk-release=1.8")
             freeCompilerArgs.add("-Xexpect-actual-classes")
         }
     }

services/build.gradle.kts

@@ -131,6 +131,7 @@ subprojects {
         compilerOptions {
             allWarningsAsErrors.set(false) // FIXME Tons of errors occur in generated code
             jvmTarget.set(JvmTarget.JVM_1_8) // fixes outgoing variant metadata: https://github.com/smithy-lang/smithy-kotlin/issues/258
+            freeCompilerArgs.add("-Xjdk-release=1.8")
         }
     }