ci: replace coveralls with codecov server

[notmandatory]

Describe the enhancement

The coveralls server has been failing recently I propose we switch to the codecov server in our CI pipeline.

https://github.com/codecov/codecov-action
https://docs.codecov.com/docs/github-tutorial

Use case

If we use the codecov GitHub action we should be able to get coverage report comments in the PRs as we do now with coveralls.

Impact

• Blocking production usage
• Nice-to-have / UX improvement
• Developer experience / maintainability

Originally posted by @tvpeter in #207 (comment)

See also: bitcoindevkit/bdk_wallet#320