prereqs Dockerfile fails to build: bullseye release file is expired

`net/grpc/gateway/docker/prereqs/Dockerfile` cannot build. Both of its stages are on Debian 11, whose release file is expired, so `apt-get update` fails with exit code 100 on the first `RUN`.

```
#10 [prepare 2/9] RUN apt-get -qq update && apt-get -qq install -y curl unzip
#10 1.940 E: Release file for http://deb.debian.org/debian-security/dists/bullseye-security/InRelease is expired (invalid since 1d 8h 2min 24s).
#10 ERROR: process "/bin/sh -c apt-get -qq update && apt-get -qq install -y curl unzip" did not complete successfully: exit code: 100
```

To reproduce:

```
docker build -f net/grpc/gateway/docker/prereqs/Dockerfile .
```

Every image that builds `FROM grpcweb/prereqs` is blocked by this: `binary_client`, `closure_client`, `commonjs_client`, `interop_client`, `node_server`, `echo_server`, `ts_prereqs`, `ts_client`.

We hit it in eclipse-vertx/vertx-grpc, which builds this image to run the gRPC-Web interop tests: https://github.com/eclipse-vertx/vertx-grpc/actions/runs/34314125183/job/102346651634

Start with net/grpc/gateway/docker/prereqs/Dockerfile and reproduce the failure with docker build -f net/grpc/gateway/docker/prereqs/Dockerfile .. Inspect both Debian 11 stages and the first apt-get update command. Done means the prereqs image builds successfully and the listed images that use grpcweb/prereqs are no longer blocked.

Written by the indexing model from the issue text.