[gems] Update simplecov and codecov

[hallelujah]

To work around the failures we see in the CircleCI...

It actually does not fix the actual issue with CodeCov Bash. We just disable it.

{'detail': ErrorDetail(string='Build has already finished, uploads rejected.', code='not_found')}
404

We did all wrong in the past:

• codecov gem has an auto upload feature (who looks like broken with our ruby/bundler)
• we also re-upload the results with the bash script given by codecov

This PR fixes that by:

• letting Ruby generate the codecov reports
• letting the circleci job to upload the generated reports to codecov

Explanation

Looks like they deployed a feature on Friday 19/02/2021 that rejects uploads
Breaking multiple concurrent builds in CI
See codecov/codecov-bash#411

Notes

• This is the kind of things that go against deploying on Friday
• We can't really use after_n_builds in the codecov.yml because we have so much parallelism and builds ... and it could change any time.