Avoid using gateway terms

[chuangw6]

Context: #68 (comment)

Not this PR, but I think, we should avoid using the gateway term. We should probably start using the term "api server" or something.
Gateway is such an overloaded term in the networking domain that it might confuse people.

Example places where we use gateway

• https://github.com/gke-labs/open-rl/blob/main/src/server/gateway.py
• https://github.com/gke-labs/open-rl/blob/main/src/server/Dockerfile.gateway
• https://github.com/gke-labs/open-rl/blob/main/docs/architecture.md