[coverage] Switch to `codecov/codecov-action`

[ValuedMammal]

Describe the enhancement

Switch from coveralls to codecov/codecov-action in .github/workflows/code_coverage.yml to start uploading coverage reports to codecov.io.

For example refer to bitcoindevkit/bdk_wallet#327.

Use case

Impact

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

[Toyin5]

Can I get assigned to this? I'd like to work on this enhancement