.github/workflows: Bump codecov-action to v2
.github/workflows: Bump codecov-action to v2

The codecov bash uploader is being replaced (supposedly partially for
security reasons, but it seems maintenance reasons are the real goal).
https://about.codecov.io/blog/codecov-uploader-deprecation-plan/

v1 uses the bash uploader. v2 uses the new uploader. The bash uploader
will begin seeing brownouts soon.

grpc/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,ejona86/grpc-java,grpc/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,stanley-cheung/grpc-java