Replace coveralls with codecov

[surchs]

Coveralls has recently become a source of flaky tests which is occasionally annoying. We could probably find ways to fix this. But the main reason we started with coveralls over codecov was because Alyssa's github account didn't play nice with Codecov - and that has since been fixed.

I personally prefer the code reports from codecov. So I propose we start migrating to codecov and replace coveralls.

[surchs]

e.g. https://github.com/neurobagel/bagel-cli/actions/runs/7197612436/job/19605132564?pr=252#step:7:79

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