build(deps): bump kotlin from 2.0.0 to 2.0.21

Bumps `kotlin` from 2.0.0 to 2.0.21.

Updates `org.jetbrains.kotlin.jvm` from 2.0.0 to 2.0.21

Updates `org.jetbrains.kotlin.plugin.serialization` from 2.0.0 to 2.0.21

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

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

gradle/libs.versions.toml

@@ -1,7 +1,7 @@
 [versions]
 jetty = "9.4.55.v20240627"
 junit = "5.11.0"
-kotlin = "2.0.0"
+kotlin = "2.0.21"
 kotlinx-coroutines = "1.9.0"
 kotlin-logging = "3.0.5"
 ktor = "2.3.12"