Update Android Kotlin documentation for RN0.73+

[adammcarth]

Summary

Some minor changes made to the README and Kotlin document to reflect that fact that React Native 0.73+ now has Kotlin included by default, and that despite this, some minor modifications to the android/build.gradle will still be required. The changes also reminds developers that they can use the existing variable instead of defining a specific Kotlin version twice.

How did you test this change?

N/A