Fix codecov upload

[sostock]

This uses the codecov/codecov-action action for uploading to codecov instead of the deprecated julia-actions/julia-uploadcodecov. Hopefully, this fixes the codecov upload that seems to have broken recently.

[giordano]

This will stop working at some point. We need to move to codecov/codecov-action@v3 and an admin of this repo to set up a codecov token.

[sostock]

It looks like it already stopped working without a token.

[giordano]

My point is that codecov/codecov-action@v3 will stop working when Nodejs v16 will be retired in github actions in a few months. We can go on with this PR right now, but later this year we'll be again without codecov working.

[sostock]

We can go on with this PR right now, but later this year we'll be again without codecov working.

I think merging this PR won’t do anything, the upload doesn’t work with this PR either. But I can change it to codecov-action@v4 (I thought v3 would work without token, that’s why I chose it). Then we only need an admin to add the token.