Use java toolchain version 21, ignore out directory

This fixes the build when running the tests with IntelliJ IDEA's
internal build tool.

Author: rhaendel

.gitignore

@@ -4,6 +4,7 @@
 !.idea/fileTemplates
 .kotlin/
 build
+out
 src/main/resources/**/*.txt
 !src/main/resources/2024/Day99.txt
 local.properties

build.gradle.kts

@@ -19,6 +19,12 @@ dependencies {
     testImplementation("com.github.stefanbirkner:system-lambda:1.2.1")
 }
 
+java {
+    toolchain {
+        languageVersion = JavaLanguageVersion.of(21)
+    }
+}
+
 tasks {
     wrapper {
         gradleVersion = "8.11.1"