Update dependency flutter_local_notifications to v21

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
flutter_local_notifications (source)	dependencies	major	^20.1.0 → 21.0.0

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Release Notes

MaikuB/flutter_local_notifications (flutter_local_notifications)

v21.0.0

Compare Source

• Breaking change bumped minimum Flutter SDK requirement to 3.38.1 and Dart SDK requirement to 3.10.0. Consequently the minimum OS requirements for each platform has been updated as well
  • [Android] minimum Android version is now 7.0 (API level 24)
  • [iOS] minimum iOS version is now 13
  • [macOS] minimum macOS version is now 10.15
• Bumped timezone dependency
• [Android] Breaking change bumped compileSdk to 36 and updated readme to mention this
• [Android] bumped Android Gradle Plugin (AGP) to 8.11.1
• Migrated example app to UIScene lifecycle
• Updated readme to specify changes needed for applications that have migrated to UIScene lifecycle
• Updated iOS-specific setup around registering UNUserNotificationCenterDelegate to remove redundant available check and to be more specific that it should be done in the application:didFinishLaunchingWithOptions: method
• Bumped flutter_timezone dependency in example app

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