Make grpc-web available on bazel central registry

grpc-web needs to be listed in the [Bazel Central Registry](https://registry.bazel.build/search?q=grpc) to be available to bzlmod users. (Users can currently work around this by using [archive_override](https://registry.bazel.build/search?q=grpc), I believe.)

Steps required:

- [ ] Make release archives stable. See https://github.com/grpc/grpc-web/pull/1464
- [ ] Submit a PR to the Bazel Central Registry per https://github.com/bazelbuild/bazel-central-registry/blob/main/docs/README.md. In progress: https://github.com/bazelbuild/bazel-central-registry/pull/2541