Fix Gradle toolchain resolution on macOS/Corretto

Replace jvmToolchain(17) with jvmToolchain(21) + explicit kotlinOptions
jvmTarget=17 + java sourceCompatibility/targetCompatibility. Avoids
toolchain provisioning errors when the JDK is present but not registered
via the Gradle toolchain API.

build.gradle.kts

@@ -10,7 +10,16 @@ group = "com.bolenpad.photophetch"
 version = "1.0.0"
 
 kotlin {
-    jvmToolchain(17)
+    jvmToolchain(21)
+}
+
+tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile> {
+    kotlinOptions.jvmTarget = "17"
+}
+
+java {
+    sourceCompatibility = JavaVersion.VERSION_17
+    targetCompatibility = JavaVersion.VERSION_17
 }
 
 dependencies {