Updated Kotlin gradle plugin version to 1.6.21

[danger-ahead]

This was the error before this fix:

Flutter Fix
[!] Your project requires a newer version of the Kotlin Gradle plugin.

java -version output:

openjdk version "1.8.0_352"
OpenJDK Runtime Environment (build 1.8.0_352-b08)
OpenJDK 64-Bit Server VM (build 25.352-b08, mixed mode)

flutter --version output:

Flutter 3.3.7 • channel stable • https://github.com/flutter/flutter.git
Framework • revision e99c9c7cd9 (3 days ago) • 2022-11-01 16:59:00 -0700
Engine • revision 857bd6b74c
Tools • Dart 2.18.4 • DevTools 2.15.0