Bump sentry-core to 0.28

[MarijnS95]

Checklist

• [x] I have read the Contributor Guide
• [x] I have read and agree to the Code of Conduct
• [x] I have added a description of my changes and why I'd like them included in the section below

Description of Changes

Upgrade to the latest available Sentry release: https://github.com/getsentry/sentry-rust/releases/tag/0.28.0

[MarijnS95]

The deny config has quite a few errors and warnings...