CI: Upgrade codecov-action to v2
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Seems benign; what does this do? The old uploader is deprecated. From the codecov github releases page.
```
On February 1, 2022, the v1 uploader will be full sunset and no longer function. This is due
to the deprecation of the underlying bash uploader. This version uses the new uploader.

The v2 Action downloads, verifies, and runs the Codecov binary.
```
Probably better to upgrade it sooner rather than later. Thanks @lithomas1 