Secure .NET Upgrade & Fixes

My production C# application currently targets .NET Framework 4.8 and has started to show its age. I need it migrated to a supported, more secure runtime (ideally the latest LTS release of .NET), with every dependency updated and any obsolete APIs refactored. The primary driver is security: I want all outstanding vulnerabilities removed, HTTPS enforcement tightened, and modern authentication/cryptography practices adopted throughout the codebase.

During the upgrade I also want to introduce a clean integration layer so the app can talk to external systems (REST and, if feasible, simple message-queue endpoints). This should be implemented in a way that keeps the core logic decoupled and unit-testable.

Finally, a handful of persistent UI glitches—visual misalignments and sporadic control flicker—need to be tracked down and resolved so the interface behaves consistently on high-DPI screens.

Acceptance criteria for the engagement:
• Solution builds and runs on the target .NET version without warnings.
• OWASP dependency-check or comparable tool reports zero critical/high issues.
• Integration module exposes well-documented interfaces and passes basic end-to-end tests against a sample external system.
• All previously noted UI issues are reproducible on 4.8, absent on the upgraded build.

Please work in a Git branch, commit frequently, and include a concise migration report outlining changes, removed packages, and any manual steps required for future deployments.