Replace codecov uploader

[ptomato]

We use codecov.io to see how much of the reference code is covered by test262, in order to get information on where there might be testing gaps.

Already several years ago, codecov.io has ended support for a bunch of their uploader packages, including codecov-node, the one we use: https://about.codecov.io/blog/codecov-uploader-deprecation-plan/

codecov-node still works for the time being. I was afraid that it had stopped working when the codecov.io upload started failing on PRs, but luckily that was solved by upgrading from 3.8.2 to 3.8.3, the very last version. However, since the package is unsupported, it will probably continue to bitrot and stop working eventually.

We should migrate to use codecov.io's GitHub Action, as described here: https://docs.codecov.com/docs/codecov-uploader

[ljharb]

I'd suggest sticking to v3.1.5 of the codecov action, since v4 requires a token, and ^3.1.6 doesn't work on node 20.