Modernize build configuration and add Swift Package Manager support

[tulioccalazans]

Overview

This PR updates fk_user_agent to improve compatibility with modern Flutter, Android, and iOS development environments.

Changes

• Upgrade the Android build configuration to modern Android Gradle Plugin and Gradle versions.
• Update Android SDK configuration and namespace handling.
• Increase the Android minimum SDK requirement to API 24 and target SDK to 36.
• Replace the deprecated JCenter repository with Maven Central.
• Update the package version to 2.1.2.
• Update project and dependency lockfiles.
• Add Swift Package Manager (SPM) support for iOS.
• Update the iOS source structure and CocoaPods configuration to support the new package layout.
• Modernize the iOS example project and update its deployment target.
• Update the iOS example to use the modern Flutter implicit engine/lifecycle integration.

Motivation

These changes modernize the plugin's build infrastructure and improve its compatibility with current Flutter and platform tooling, while also adding Swift Package Manager support for iOS projects.

No changes are intended to the plugin's public Dart API or its core user-agent functionality.