gh-35544: Add "informational: true" to .codecov.yml
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### :books: Description
Add `informational: true` to `.codecov.yml` to avoid "codecov/project"
check failure, as discussed in
https://github.com/sagemath/sage/issues/35522#issuecomment-1512287814
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/35544
Reported by: Kwankyu Lee
Reviewer(s): Matthias Köppe
