Fix incorrect usage of WidgetFlutterBindings.ensureInitialized()

[matux]

Description of the change

Fixes some deprecation issues, and an incorrect usage of WidgetFlutterBindings.ensureInitialized() on Flutter 3.10.x:

The Flutter bindings were initialized in a different zone than is now being used. This will likely cause confusion and bugs as any zone-specific configuration will inconsistently use the configuration of the original binding initialization zone or this zone based on hard-to-predict factors such as which zone was active when a particular callback was set.
It is important to use the same zone when calling ensureInitialized on the binding as when calling runApp later.

• 126798

This was reported https://github.com/rollbar/rollbar-flutter/issues/90#issuecomment-1568424569 and https://github.com/rollbar/rollbar-flutter/issues/90#issuecomment-1569633188.

Type of change

• [x] Bug fix (non-breaking change that fixes an issue)
• [ ] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Maintenance
• [ ] New release

Related issues

This is being partially tracked here: https://github.com/rollbar/rollbar-flutter/issues/90, but the issue itself is about something else, so this wouldn't close the issue.

Checklists

Development

• [x] Lint rules pass locally
• [x] The code changed/added as part of this pull request has been covered with tests
• [x] All tests related to the changed code pass in development

Code review

• [x] This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached
• [x] "Ready for review" label attached to the PR and reviewers assigned
• [x] Issue from task tracker has a link to this pull request
• [ ] Changes have been reviewed by at least one other engineer