Bump kotlin.version from 2.1.20 to 2.1.21

Bumps `kotlin.version` from 2.1.20 to 2.1.21.

Updates `org.jetbrains.kotlin:kotlin-stdlib` from 2.1.20 to 2.1.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.1.20...v2.1.21)

Updates `org.jetbrains.kotlin:kotlin-maven-plugin` from 2.1.20 to 2.1.21

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin:kotlin-stdlib
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin:kotlin-maven-plugin
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

pom.xml

@@ -26,7 +26,7 @@
         <url>https://github.com/javadev/LeetCode-in-Kotlin</url>
     </scm>
     <properties>
-        <kotlin.version>2.1.20</kotlin.version>
+        <kotlin.version>2.1.21</kotlin.version>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
     </properties>
     <ciManagement>