fix(deps): update dependency org.jetbrains.kotlin:kotlin-gradle-plugin to v2.3.21

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
org.jetbrains.kotlin:kotlin-gradle-plugin (source)	2.3.20 → 2.3.21

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[codacy-production]

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[github-actions]

APK file: https://github.com/nextcloud/notes-android/actions/runs/24825645700/artifacts/6597481341
To test this change/fix you can simply download above APK file and install and test it in parallel to your existing Nextcloud app.
(please click on link to get QR code displayed)