Consider renaming HAProxy API backends

The naming of our API backends in HAProxy is confusing. api_rate_limit does not mean that an API call has been rate limited, but rather than it's not exempt from rate limiting. This may lead to SREs assuming, when they receive pagerduty alerts for the HAProxy api_rate_limit backend that the alert is being generated because a user is exceeding our rate limits, and as such, the alert can be ignored.

This is not the case.

The problem is that the name of this backend is misleading. Most external requests are passed through this backend.

We should consider renaming these backends, to something such as:

• api -> api_rate_limiting_exempt. No rate limiting is applied for specific allow-list (GitLab CI, some marquee customers, etc)
• api_rate_limit -> api_rate_limiting_applicable. Rate limiting will be applied to these clients by default.
• 429_slow_down -> api_rate_limit_tar_pit. Clients that are actively being slowed down.

Naming is hard, other suggestions would be appreciated.

cc @skarbek @devin @dawsmith @AnthonySandoval @albertoramos