Update default api endpoint

[gzussa]

Update default endpoint from app to api

[Viq111]

To give more color to the PR: https://app.datadoghq.com/ is the endpoint used for calls coming from the web (i.e: usually from a web browser).

The https://api.datadoghq.com/ address has a thinner load balancer layer (less routes and more consistent traffic) so overall better availability for API calls

[gzussa]

cc @jkschneider and @shakuzen since i cannot add you as reviewer 😄

[shakuzen]

Thank you for the pull request. And thank you for the explanation of the motivation @Viq111 - this is good to have for documenting the change.