build: bump the gradle group with 4 updates

Bumps the gradle group with 4 updates: [org.jetbrains.kotlin:kotlin-stdlib](https://github.com/JetBrains/kotlin), [org.jetbrains.kotlin:kotlin-test](https://github.com/JetBrains/kotlin), [org.jetbrains.kotlin.multiplatform](https://github.com/JetBrains/kotlin) and [org.jetbrains.kotlin.plugin.serialization](https://github.com/JetBrains/kotlin).


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-test` 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.multiplatform` 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.plugin.serialization` 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-test` 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.multiplatform` 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.plugin.serialization` 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)

---
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-group: gradle
- dependency-name: org.jetbrains.kotlin:kotlin-test
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: gradle
- dependency-name: org.jetbrains.kotlin.multiplatform
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: gradle
- dependency-name: org.jetbrains.kotlin.plugin.serialization
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: gradle
- dependency-name: org.jetbrains.kotlin:kotlin-test
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: gradle
- dependency-name: org.jetbrains.kotlin.multiplatform
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: gradle
- dependency-name: org.jetbrains.kotlin.plugin.serialization
  dependency-version: 2.1.21
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: gradle
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

gradle/libs.versions.toml

@@ -1,5 +1,5 @@
 [versions]
-kotlin-stdlib = "2.1.20"
+kotlin-stdlib = "2.1.21"
 kotlin-serialization = "1.8.1"
 android-gradle = {strictly = "8.7.3"}
 dokka = "2.0.0"