Prepare for v2.0.0

[emcfarlane]

This prepares for the v2.0.0 release of connectrpc.com/grpchealth/v2. Below are the proposed release notes:

What's new in version 2

grpchealth-go v2 is built for connect-go v2. The module path is now connectrpc.com/grpchealth/v2. Health-checking behavior, error codes, and wire compatibility with Google's gRPC implementations are unchanged. v1 is still supported for connect-go v1, and because the module paths differ, you can use both in the same module.

The health service now registers on a connect.Server instead of returning an http.Handler, and the client takes a *connect.Client:

-mux.Handle(grpchealth.NewHandler(checker))
+connectServer := connect.NewServer()
+grpchealth.Register(connectServer, checker)
+connecthttp.Mount(mux, connectServer)

-client := grpchealth.NewClient(httpClient, "https://api.example.com", connect.WithGRPC())
+client := grpchealth.NewClient(connect.NewClient(connecthttp.NewTransport(
+     httpClient,
+     "https://api.example.com",
+     connecthttp.WithGRPC(),
+)))

Checker, Watcher, StaticChecker, CheckRequest, CheckResponse, Status, and WatchEvent are unchanged. The v2 migration guide covers the change, and the connect-go-v2-migrate tool rewrites both forms automatically. See #89 for details.

To upgrade, run:

go get connectrpc.com/grpchealth/v2@v2.0.0

Full Changelog: v1.5.0...v2.0.0