CodeCov Requires Token

[KyleKing]

Context

Previously, the template and all plugins included a CodeCov upload step, however the secret token has expired or was never configured, which has caused CI to fail across repositories.

See: https://github.com/executablebooks/mdformat-myst/actions/runs/10509081678/job/29114425723

{'detail': ErrorDetail(string='Rate limit reached. Please upload with the Codecov repository upload token to resolve issue. Expected time to availability: 2352s.', code='throttled')}

Proposal

We can either drop support for CodeCov or someone with the proper permissions (I can't) must add a Token.

CodeCov can be useful for reviewers to complete code review on the Web without checking out the code locally to test coverage or digging through the Github Action logs, but I don't think it is strictly necessary and could be removed.

For now, I've commented out the broken Github Action step to unblock CI and releases: https://github.com/executablebooks/mdformat-myst/pull/29/files