Update API Rate Limit Headers to reflect IETF Proposal

[tpaulus]

Summary

The underlying services were updated one to two weeks ago to use the IETF proposed Ratelimit and Ratelimit-Policy headers. This change updates the relevant Changelog announcing the new headers and the Fundamentals section to more accurately reflect the response headers that relate to rate limiting.

Full draft IETF proposal: https://ietf-wg-httpapi.github.io/ratelimit-headers/draft-ietf-httpapi-ratelimit-headers.html#name-introduction

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Documentation checklist

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