Bump org.jetbrains.kotlinx:atomicfu from 0.20.2 to 0.21.0

Bumps [org.jetbrains.kotlinx:atomicfu](https://github.com/Kotlin/kotlinx.atomicfu) from 0.20.2 to 0.21.0.
- [Release notes](https://github.com/Kotlin/kotlinx.atomicfu/releases)
- [Changelog](https://github.com/Kotlin/kotlinx-atomicfu/blob/master/CHANGES.md)
- [Commits](Kotlin/kotlinx-atomicfu@0.20.2...0.21.0)

---
updated-dependencies:
- dependency-name: org.jetbrains.kotlinx:atomicfu
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

gradle/libs.versions.toml

@@ -4,7 +4,7 @@
 
 jackson = "2.15.2"
 jetty-websocket = "9.4.51.v20230217"
-kotlinx-atomicfu = "0.20.2"
+kotlinx-atomicfu = "0.21.0"
 kotlinx-coroutines = "1.7.3"
 kotlinx-serialization = "1.5.1"
 ktor = "2.3.1"