Bump jvm from 1.9.20 to 1.9.21 in /samples/jvm/junit5

Bumps [jvm](https://github.com/JetBrains/kotlin) from 1.9.20 to 1.9.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v1.9.20...v1.9.21)

---
updated-dependencies:
- dependency-name: jvm
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

samples/jvm/junit5/build.gradle.kts

@@ -7,7 +7,7 @@ val atriumVersion = "1.1.0"
 val junitVersion = "5.10.1"
 
 plugins {
-    kotlin("jvm") version "1.9.20"
+    kotlin("jvm") version "1.9.21"
 }
 
 group = "org.atriumlib.samples"