chore: upgrade Gradle, AGP, Kotlin and fix build errors for new contributors

[Prathvi-Raj-Chauhan]

Problem

New contributors were unable to build the project due to outdated
Gradle, AGP, and Kotlin versions causing compilation errors.

Changes

• Gradle: 8.4 -> 8.9
• AGP: 8.2.1 -> 8.7.0
• Kotlin: 1.9.22 -> 2.1.0
• compileSDK: 34 -> 36
• awesome_notifications: 0.10.0 -> 0.11.0 (fixes PluginRegistrantCallback error)
• minSdkVersion: hardcoded 23 -> flutter.minSdkVersion (Flutter convention)
• .gitignore: added .gradle/ to prevent build cache from being tracked

Tested on

• Flutter: 3.41.9
• Android emulator: sdk gphone64 x86 64

[netlify]

❌ Deploy Preview for aquamarine-kheer-83feda failed. Why did it fail? →

Name	Link
🔨 Latest commit	ba4338b
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[github-actions]

Thank you for submitting your pull request! We'll review it as soon as possible.

[Prathvi-Raj-Chauhan]

@andoriyaprashant awesome_notifications is under dev_dependencies, shouldn't it be under dependencies since it's used at runtime ?