Port to new codecov configuration

[azeey]

🦟 Bug fix

Summary

Use the codecov config proposed in gazebo-tooling/action-gz-ci#32.
This also uses a new codecov url in the README now that sdformat is in the ignitionrobotics org.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge

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[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (sdf9@33fc40a). Click here to learn what that means.
The diff coverage is n/a.

❗ Current head 34bc0d7 differs from pull request most recent head 7870e79. Consider uploading reports for the commit 7870e79 to get more accurate results

@@           Coverage Diff           @@
##             sdf9     #676   +/-   ##
=======================================
  Coverage        ?   86.83%           
=======================================
  Files           ?       62           
  Lines           ?     9734           
  Branches        ?        0           
=======================================
  Hits            ?     8453           
  Misses          ?     1281           
  Partials        ?        0           

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