Upgrade dsl plugin. (firebase#2772)

Gradle 5.6.4 expects the plugin to be version 1.2.9.

Author: rlazo

buildSrc/build.gradle

@@ -13,7 +13,7 @@
 // limitations under the License.
 
 plugins {
- id "org.gradle.kotlin.kotlin-dsl" version "1.2.6"
+ id "org.gradle.kotlin.kotlin-dsl" version "1.2.9"
  id "org.jlleitschuh.gradle.ktlint" version "9.2.1"
  id 'com.github.sherter.google-java-format' version '0.9'
 }