Tests rely on out-of-date envoy

[aran]

The tests use envoy 1.19.

https://github.com/grpc/grpc-dart/blob/master/.github/workflows/dart.yml#L82

test/grpc_web_server.dart uses a configuration file that is deprecated:

grpc-dart/test/grpc_web_server.dart

Line 83 in 9a9c017

- name: envoy.filters.http.grpc_web

Modern versions of envoy require a change like this:

grpc/grpc-web#1220 (comment)