neutron: update build to track understack/2026.1 at upstream branch

I had previously cherry-picked the upstream evpn driver but it wasn't
doing what was needed so it is now replaced by our own driver.

Therefore, we can remove the evpn commits and reset to vanilla upstream.

Author: stevekeay

containers/neutron/Dockerfile

@@ -6,7 +6,7 @@ FROM quay.io/airshipit/neutron:${OPENSTACK_VERSION}-ubuntu_noble AS build
 COPY --from=ghcr.io/astral-sh/uv:latest /uv /uvx /bin/
 
 # renovate: name=openstack/neutron repo=https://github.com/rackerlabs/neutron.git branch=understack/2026.1
-ARG NEUTRON_GIT_REF=f02658b3721de198f2dac04e7eb64ef9eb320732
+ARG NEUTRON_GIT_REF=2bc474a7db20f837cabfd60de1ebd3fa0c51f736
 ADD --keep-git-dir=true https://github.com/rackerlabs/neutron.git#${NEUTRON_GIT_REF} /src/neutron
 RUN git -C /src/neutron fetch --unshallow --tags