Update feedback_sentry to use sentry: ^8.0.0

[felipheallef]

:scroll: Description

This PR updates pubspec.yaml file for the feedback_sentry setting the version constraint of sentry to ^8.0.0. Although the changes in sentry: ^8.0.0 don't affect this packages they might affect apps using it so I also updated the version of this package to 4.0.0.

:bulb: Motivation and Context

The Sentry team released version 8.0.0 of the sentry package on Apr 22, 2024. As it is a new major version, packages depending on it needs to update its version constraints. Closes #301.

:green_heart: How did you test it?

Running the test with flutter test:

flutter test test/feedback_sentry_test.dart     
00:01 +1: All tests passed! 

:pencil: Checklist

• [x] I reviewed submitted code
• [x] I added tests to verify changes
• [x] I updated the docs if needed
• [x] All tests passing
• [x] No breaking changes

:crystal_ball: Next steps