upgrade to Kotlin 1.9.21

[buenaflor]

📜 Description

Update the SDK and the samples to 1.9.21

No changes in the src files, only added opt-in annotations that were flagged.

I had to rearrange files to the new sourceSets so that why there's lots of files changed

💡 Motivation and Context

Closes #137

💚 How did you test it?

📝 Checklist

You have to check all boxes before merging:

• I reviewed the submitted code.
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.

🔮 Next steps

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (add53d3) 77.83% compared to head (f00e165) 77.83%.

❗ Current head f00e165 differs from pull request most recent head 59a9526. Consider uploading reports for the commit 59a9526 to get more accurate results

Files	Patch %	Lines
...tform/extensions/SentryAndroidOptionsExtensions.kt	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #146   +/-   ##
=======================================
  Coverage   77.83%   77.83%           
=======================================
  Files          27       27           
  Lines         424      424           
  Branches       55       55           
=======================================
  Hits          330      330           
  Misses         68       68           
  Partials       26       26           

☔ View full report in Codecov by Sentry.
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[romtsn]

lgtm!

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

- upgrade to Kotlin 1.9.21 ([#146](https://github.com/getsentry/sentry-kotlin-multiplatform/pull/146))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

Generated by 🚫 dangerJS against 59a9526