Merge bitcoindevkit/bdk-cli#247: CI: Replace Coveralls with Codecov for coverage...

a675935153a95adc0c07da5614a57cf5a1c3f628 CI: Update code coverage to use Codecov action (Vihiga Tyonum)

Pull request description:

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  ### Description

  This PR updates the code_coverage workflow by replacing the Coveralls code coverage reports with codecov. It also updates the checkout, rust-lang toolchain setup actions.
  Fixes #245

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  ### Notes to the reviewers

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  ## Changelog notice

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  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk-cli/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  notmandatory:
    ACK a675935153a95adc0c07da5614a57cf5a1c3f628

Tree-SHA512: 021e17bfec6b4c022b8908f2dc4588c3f9f809893d2c6599a073fa943d7caf96a46a47fc9bc54af701b003b95d0b4e1474e9c30fdb533570b3a1d8791f430ba7