fix(core): fix kotlin gradle version for integrations

[bsneed]

• Fixes The Android Gradle plugin supports only Kotlin Gradle plugin version 1.3.10 and higher. The following dependencies do not satisfy the required version: project ':@segment_analytics-react-native-firebase' -> org.jetbrains.kotlin:kotlin-gradle-plugin:1.3.0 #90
• Changed integration template to use Gradle 1.3.21

[fathyb]

LGTM! nitpicks:

• Any reason to use 1.3.21 instead of 1.3.50?
• Could you add which issues this fixes in the PR description and make sure it is in the commit description when merged? This will ensure the changelog is properly generated

[fathyb]

Nevermind for the version! Just noticed we're already using 1.3.21 in the main module:

analytics-react-native/packages/core/android/build.gradle

Line 11 in 9511c98

classpath 'org.jetbrains.kotlin:kotlin-gradle-plugin:1.3.21'