Since nginx 1.27.3, we can make use of the `resolve` parameter for an `upstream`'s `server`,
to allow DNS resolution to happen continuously at runtime, not just once during startup.

Previously, this was not possible to do in an `upstream` block without
an nginx-plus subscription. Outside of an `upstream` block, we've used
and still use `set $backend ..` workarounds to get DNS resolution at
runtime, but now we can do it in `upstream` as well.