ci: integrate codecov

[libotony]

Also added go 1.21.x to GitHub action flow

[codecov-commenter]

Welcome to Codecov 🎉

Once merged to your default branch, Codecov will compare your coverage reports and display the results in this comment.

Thanks for integrating Codecov - We've got you covered ☂️

[qianbin]

1.18 can be abandoned

[darrenvechain]

@libotony should we enforce a minimum threshold in the CI ?

[libotony]

@libotony should we enforce a minimum threshold in the CI ?

Do you mean the threshold for code coverage?

[darrenvechain]

@libotony should we enforce a minimum threshold in the CI ?

Do you mean the threshold for code coverage?

Ya exactly

[libotony]

@libotony should we enforce a minimum threshold in the CI ?

Do you mean the threshold for code coverage?

Ya exactly

I don't know how but it's a good to-have feature, we can set it after we top up the overall coverage.