Bump kotlin from 2.0.20 to 2.0.21

Bumps `kotlin` from 2.0.20 to 2.0.21.

Updates `org.jetbrains.kotlin.plugin.serialization` from 2.0.20 to 2.0.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.0.21/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.0.20...v2.0.21)

Updates `org.jetbrains.kotlin.multiplatform` from 2.0.20 to 2.0.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.0.21/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.0.20...v2.0.21)

Updates `org.jetbrains.kotlin.native.cocoapods` from 2.0.20 to 2.0.21
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/v2.0.21/ChangeLog.md)
- [Commits](JetBrains/kotlin@v2.0.20...v2.0.21)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlin.plugin.serialization
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin.multiplatform
  dependency-type: direct:production
  update-type: version-update:semver-patch
- dependency-name: org.jetbrains.kotlin.native.cocoapods
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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

Author: dependabot[bot]

gradle/libs.versions.toml

@@ -8,7 +8,7 @@ firebase-bom = "33.2.0"
 gitlive-firebase-java-sdk = "0.4.6"
 gson = "2.11.0"
 junit = "4.13.2"
-kotlin = "2.0.20"
+kotlin = "2.0.21"
 kotlinx-coroutines = "1.9.0-RC.2"
 kotlinx-serialization = "1.7.2"
 kotlinx-binarycompatibilityvalidator = "0.16.3"