coverage.yml: updated codecov/codecov-action to v3

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v1 has been deprecated and is considered "no longer function[ing]" since February - see https://github.com/codecov/codecov-action#%EF%B8%8F--deprecation-of-v1

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As a side note. I think we could limit the coverage generation to the main branch. We have not integrated this with PRs (if even possible) and I guess nobody is using this to check the coverage before a PR is being flagged as "ready". Also the external and test folder should be excluded. And looking at the report it seems like some data is missing - although all necessary info about the coverage is there.