Bump kotlin-gradle-plugin from 1.6.20 to 1.6.21

Bumps [kotlin-gradle-plugin](https://github.com/JetBrains/kotlin) from 1.6.20 to 1.6.21.
- [Release notes](https://github.com/JetBrains/kotlin/releases)
- [Changelog](https://github.com/JetBrains/kotlin/blob/master/ChangeLog.md)
- [Commits](https://github.com/JetBrains/kotlin/commits)

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

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

quicktype-plugin/build.gradle

@@ -11,7 +11,7 @@ description = "Collection of Quicktype related Gradle plugins"
 dependencies {
     implementation "org.jetbrains.kotlin:kotlin-reflect"
     implementation "com.github.node-gradle:gradle-node-plugin:3.2.1"
-    implementation "org.jetbrains.kotlin:kotlin-gradle-plugin:1.6.20"
+    implementation "org.jetbrains.kotlin:kotlin-gradle-plugin:1.6.21"
 
     testImplementation project(":test-common")
     testImplementation "org.jetbrains.kotlin:kotlin-test"