Use new declarative way of Gradle plugin

[kackogut]

Description

Change depreacated Gradle Plugin to remove build warnings, which will soon be changed to errors: https://docs.flutter.dev/release/breaking-changes/flutter-gradle-plugin-apply

Fixes #283

Checklist:

Creator

• [ x] The target branch is main
• I have updated the unreleased section of the change log
• [ x] I have reviewed the 'files changed' tab on github to ensure all changes are as expected
• [ x] I have added someone to review the pr

Reviewer

• I have verified the above are all completed
• I have run the code locally to ensure it fufills the requirements of the issue
• I have reviewed the 'files changed' and commented on any sections which I think are not needed, incorrect or could be improved

After pull

• If appropriate I have closed the issue/ moved the trello card