Bump io.swagger.core.v3.swagger-gradle-plugin in /Plan (#3533)

Bumps io.swagger.core.v3.swagger-gradle-plugin from 2.2.20 to 2.2.21.

---
updated-dependencies:
- dependency-name: io.swagger.core.v3.swagger-gradle-plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

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Plan/common/build.gradle

@@ -4,7 +4,7 @@ import org.apache.tools.ant.filters.ReplaceTokens
 plugins {
     id "dev.vankka.dependencydownload.plugin" version "$dependencyDownloadVersion"
     id "com.github.node-gradle.node" version "7.0.2"
-    id "io.swagger.core.v3.swagger-gradle-plugin" version "2.2.20"
+    id "io.swagger.core.v3.swagger-gradle-plugin" version "2.2.21"
 }
 
 configurations {