Update README: remove Sonarqube badge, update Coveralls badge

[phoenix-server]

Changes needed

1. Remove the Sonarqube badge from README.md
2. Update the Coveralls badge with:

[![Coverage Status](https://coveralls.io/repos/github/cameri/nostream/badge.svg?branch=main)](https://coveralls.io/github/cameri/nostream?branch=main)

Rationale

• Sonarqube badge is no longer maintained/relevant
• Coveralls badge needs to point to the correct branch and repository