Update coverage report action

[kuhnroyal]

The new version now supports listing individual changed files, not just packages and adds a flag that hides all files without changes to the coverage. We use that for the PR comment.

With VannaDii/pub-dev#9 the cobertura package now also reports the correct line coverage.

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary)
• I have run the tests without failures
• I have updated the CHANGELOG.md in the corresponding package

Additional context and info (if any)

[kuhnroyal]

The test failures are unrelated. Not sure whats going on there.

[kuhnroyal]

This will hopefully work once #2221 is on the default branch.

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
plugins/http2_adapter/lib/src/connection_manager_imp.dart	🟠 60.47%	🟢 79.84%	🟢 19.37%
Overall Coverage	🟢 80.58%	🟢 81.41%	🟢 0.83%

Minimum allowed coverage is 0%, this run produced 81.41%

[kuhnroyal]

Not sure what happened to the coverage comment. It shows changes in one file. We will have to keep an eye out for that. But seems to work in general.