Bump go-plugin-api from 19.1.0 to 20.4.0

Bumps [go-plugin-api](https://github.com/gocd/gocd) from 19.1.0 to 20.4.0.
- [Release notes](https://github.com/gocd/gocd/releases)
- [Commits](gocd/gocd@19.1.0...20.4.0)

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

build.gradle

@@ -63,7 +63,7 @@ sourceSets {
 
 dependencies {
   compile group: 'cd.go.plugin.base', name: 'gocd-plugin-base', version: '0.0.1'
-  compileOnly group: 'cd.go.plugin', name: 'go-plugin-api', version: '19.1.0'
+  compileOnly group: 'cd.go.plugin', name: 'go-plugin-api', version: '20.4.0'
   compile group: 'com.bettercloud', name: 'vault-java-driver', version: '4.0.0'
   compile group: 'com.google.code.gson', name: 'gson', version: '2.8.5'