Fix Kotlin version preventing build of example on Android

[amantoux]

─ Flutter Fix ────────────────────────────
[!] Your project requires a newer version of the Kotlin Gradle plugin.
Find the latest version on https://kotlinlang.org/docs/releases.html#release-details, then update 
/Users/.../fleather/packages/fleather/example/android/build.gradle:
ext.kotlin_version = '<latest-version>'

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (23422a2) 84.25% compared to head (abc93ae) 84.25%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #183   +/-   ##
=======================================
  Coverage   84.25%   84.25%           
=======================================
  Files          60       60           
  Lines        9408     9408           
=======================================
  Hits         7927     7927           
  Misses       1481     1481           

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