Disable Codecov patch/project checks in CI/CD

[Splines]

Changes

• We disable the patch/project checks of Codecov in our pipeline. I feel it's unrealistic that we will test everything. So Codecov helps us with a nice report in the comments of a PR. However, maybe we don't want the checks (e.g. you can't merge if you haven't tested every single Ruby line) to block us and instead only use them as information.
• We move the .codecov.yml file back to the root of the repository. Unfortunately, the codecov_yml_path is buggy, see this issue: "Ignore paths" not working with custom Codecov config file location codecov/codecov-action#1465. Once that is resolved we may switch back to the new .config folder location. Having placed the config file into the root, Codecov now correctly reads the file and also ignores the spec folder. Note that the Codecov report is only generated once the test job was executed in GitHub Actions.

---

• Docs for Codecov status checks.
• Validity of schema checked via:

curl --data-binary @.codecov.yml https://codecov.io/validate

• This PR is used in favor of Fix wrong naming of codecov config file #649.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 53.09%. Comparing base (6bde35d) to head (87f2ad2).

Additional details and impacted files

@@             Coverage Diff             @@
##              dev     #664       +/-   ##
===========================================
- Coverage   66.64%   53.09%   -13.55%     
===========================================
  Files         308      153      -155     
  Lines        9386     6614     -2772     
===========================================
- Hits         6255     3512     -2743     
+ Misses       3131     3102       -29     

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