feat: bump default kotlin to version 1.7.21

[breautek]

Platforms affected

Android / Kotlin plugins

Motivation and Context

Closes #1564

The default should be a modern version, or fairly modern version.

The chosen version is based on the Gradle Compatibility Matrix: https://docs.gradle.org/current/userguide/compatibility.html

1.7.21 is the current latest version of the 1.7 Kotlin release, based on Kotlin's Releases page.

Uses can still change the version if necessary using:

<preference name="GradlePluginKotlinVersion" value="..."/>

Why not 1.7.22?

This is a technical release. It doesn't contain any fixes that aren't included in Kotlin 1.7.21.
Version 1.7.22 of the Kotlin plugin will not be available for downloading or installing in any IDEs.

https://github.com/JetBrains/kotlin/releases/tag/v1.7.22

Description

Updated the defaults file.

Testing

Ran npm test

Checklist

• I've run the tests to see all new and existing tests pass
• I added automated test coverage as appropriate for this change
• Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• I've updated the documentation if necessary

[codecov-commenter]

Codecov Report

Merging #1594 (8bc53d3) into master (a9d4d4e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1594   +/-   ##
=======================================
  Coverage   71.82%   71.82%           
=======================================
  Files          23       23           
  Lines        1796     1796           
=======================================
  Hits         1290     1290           
  Misses        506      506           

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