docs: update coverage badge
Closes
<p align="left">
<a href="https://circleci.com/gh/ory/hydra/tree/master"><img src="https://circleci.com/gh/ory/hydra/tree/master.svg?style=shield" alt="Build Status"></a>
- <a href="https://coveralls.io/github/ory/hydra?branch=master"> <img src="https://coveralls.io/repos/ory/hydra/badge.svg?branch=master&service=github" alt="Coverage Status"></a>
+ <a href="https://codecov.io/gh/ory/hydra"><img src="https://codecov.io/gh/ory/hydra/branch/master/graph/badge.svg?token=y4fVk2Of8a"/></a>
<a href="https://goreportcard.com/report/github.com/ory/hydra"><img src="https://goreportcard.com/badge/github.com/ory/hydra" alt="Go Report Card"></a>
<a href="https://pkg.go.dev/github.com/ory/hydra"><img src="https://pkg.go.dev/badge/www.github.com/ory/hydra" alt="PkgGoDev"></a>
<a href="https://bestpractices.coreinfrastructure.org/projects/364"><img src="https://bestpractices.coreinfrastructure.org/projects/364/badge" alt="CII Best Practices"></a>
