Bump plugin.serialization from 1.6.20 to 1.6.21

Bumps plugin.serialization from 1.6.20 to 1.6.21.

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

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

build.gradle.kts

@@ -1,6 +1,6 @@
 plugins {
     kotlin("multiplatform") version "1.6.21" apply false
-    kotlin("plugin.serialization") version "1.6.20" apply false
+    kotlin("plugin.serialization") version "1.6.21" apply false
     `maven-publish`
     signing
     id("io.github.gradle-nexus.publish-plugin") version "1.1.0"