Remove service/rpc1

[derekparker]

The service/rpc2 package was introduced way back in 2016. I think 7 years is long enough to keep both around for the users sake.

I think we should remove the service/rpc1 package and make service/rpc2 just service/rpc and the default. We could keep the command line flag for a bit to avoid breaking clients, but we should deprecate it and also plan to eventually remove it as well.

If we find we really need to version the rpc package perhaps it belongs as a standalone package versioned via its own Go module.

[aarzilli]

Do we deprecate in 1.21 and delete in 1.22? Or 1.23? I'd leave rpc2 as rpc2, in case we ever do rpc3.

[derekparker]

@aarzilli I think deprecate in 1.21 and then remove in 1.23, give 2 releases worth of notice.