Use the latest protobuf tools and libraries throughout

Previously we were using a mixture of old and new, with our code generation
using the plugin from the old github.com/golang/protobuf library but
our callers using the modern google.golang.org/protobuf . We were also
using pretty ancient version of protoc.

This brings us up to the current latest releases and consistently using
the new Go protobuf library. There have been some notable changes to these
tools in the meantime:

Previously the protoc-gen-go plugin handled grpc by having its own
additional level of Go-specific "plugins" of which the gRPC codegen was
an example.

Now the protobuf generator and the gRPC generator are separate plugins
handled directly by protoc, which means the command line arguments are
a different shape and the gRPC stubs get generated in a separate file
from the main protobuf messages, rather than all being in one .pb.go file
as before.The results are otherwise similar, though.

The grpc codegen now also defaults to requiring that implementations embed
the generated "unimplemented" server, which is an implementation of each
service where the methods just immediately return the "unimplemented"
error. This is not super important for us because we maintain the generated
interfaces and their implementations together in the same repository
anyway, but adding the "unimplemented" server embeds was not a big change
and so seems better to follow the prevailing convention.

Using these new versions means that we could in principle now switch to
using protobuf edition 2024 and the new "sealed" style for Go code
generation, but this commit does not include any such changes and focuses
only on getting things upgraded with as few other changes as possible. We
can discuss using different codegen style later and deal with that in
separate commits.

Signed-off-by: Martin Atkins <mart@degeneration.co.uk>

internal/grpcwrap/provisioner.go

@@ -31,6 +31,8 @@ func Provisioner(p provisioners.Interface) tfplugin5.ProvisionerServer {
 type provisioner struct {
 	provisioner provisioners.Interface
 	schema      *configschema.Block
+
+	tfplugin5.UnimplementedProvisionerServer
 }
 
 func (p *provisioner) GetSchema(_ context.Context, req *tfplugin5.GetProvisionerSchema_Request) (*tfplugin5.GetProvisionerSchema_Response, error) {