Gradle 8 compatibility

[evil159]

What does this pull request do?

This PR makes plugin's build.gradle to be compatible with Gradle 8 by specifying the namespace attribute. Also Example's app Gradle version is bumped to Gradle 8.

What is the motivation and context behind this change?

https://github.com/mapbox/mapbox-maps-flutter/issues/217, https://mapbox.atlassian.net/browse/MAPSFLT-204

Pull request checklist:

• [x] Add a changelog entry.
• [ ] Write tests for all new functionality. If tests were not written, please explain why. Not applicable.
• [ ] Add documentation comments for any added or updated public APIs.