chore: Increase coverage target from 20% to 98%

[ianna]

as discussed at the meeting

[github-actions]

The documentation preview is ready to be viewed at http://preview.awkward-array.org.s3-website.us-east-1.amazonaws.com/PR3897

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.65%. Comparing base (7911123) to head (1b44731).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

see 2 files with indirect coverage changes

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[ianna]

@ariostas and @ikrommyd - I think I'm happy with the final configuration. Ignoring a few paths which cannot be checked (no GPU, nor ROOT) will significantly increase our current coverage. I'd like to see how close we are to 💯

[ariostas]

Seems good to me. We'll see how painful it ends up being trying to hit the 98% mark. It it's only hard sometimes we can just ignore the failure in those cases, but if it regularly happens that the tests needed are too convoluted we can just decrease it a bit.