Update Codecov action to v3

[RobbieMcKinstry]

Description

This PR updates the codecov GH Action to v3 from v2. In addition to keeping our action deps fresh, this PR will allow the GH Action to automatically use upstream releases. i.e. if a new release is made to the v3 line, we'll pick up that release automatically. I don't think the Codecov team is actively backporting bug fixes to v2.

In particular, I'm opening this PR to push closer to a resolution for #9730. I opened codecov-actions #769 to resolve #9730. If/when my PR is merged and a new release is cut, we'll pick up the new 3.2 release automatically, allowing us to close #9730.

Checklist

• I have added tests that prove my fix is effective or that my feature works

• I have updated the CHANGELOG-PENDING file with my change

• Yes, there are changes in this PR that warrants bumping the Pulumi Service API version

[AaronFriel]

If I'm reading this correctly, you'd merge this after the PR to codecov is merged?

[RobbieMcKinstry]

@AaronFriel Ah, sorry, let me clarify.
We can merge whenever. Right now the release is v3.1, so merging this PR will bump us to the latest release.
Presumably, there's some benefit to being on the latest stable release, but it's not germane to any of our open issues. It's just nice to be up to date. Since we only pin to the latest major version (v2 or v3), we'll immediately pick up v3.1.

However, if my patch gets merged, and the codecov team cuts a v3.2 release, then pulumi/pulumi will immediately pick up v3.2 with my patch in it if this PR is merged in. That will allow us to close #9730 .
If its not merged in, then the cutting of upstream release won't affect us, since we're pinned to v2.

If you want, we can wait to merge this PR, but I won't particularly see a benefit to continuing to use v2. There's no major difference that I'm aware of, so we might as well merge now and jump to v3.

[t0yv0]

Rebase on master should get CI to green.