Drop node-fetch and only use native fetch

[frederikprijck]

Changes

Builds on top of #906 to drop node-fetch altogether given we will only support node18+, which supports native fetch.

Additionally, given nock does not support native fetch, we still use node-fetch when running our tests that rely on nock.

References

nock/nock#2397

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors