Update default local version code (#504)

I forgot to update the default version code for local build in #503.
Before, devs usually get 30009999. This PR updates the version to be
90009999 to make it consistent with CI build number



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slack-plugin/src/main/kotlin/slack/gradle/ApkVersioningPlugin.kt

@@ -126,7 +126,7 @@ internal class ApkVersioningPlugin : Plugin<Project> {
 
 private object ApkVersioning {
 
-  const val DEFAULT_VERSION_CODE: Int = 9999
+  const val DEFAULT_VERSION_CODE: Int = 90009999
 }
 
 /**