Regenerate Protocol Buffers Go code to remove dependency on deprecated Go module (#140)

Reference: golang/protobuf#1306

This does not appear to be generating any meaningful differences.

Updated via:

```console
pushd tfprotov5/internal/tfplugin5
./generate.sh
popd
pushd tfprotov6/internal/tfplugin6
./generate.sh
popd
go mod tidy
```

Author: bflad

go.mod

@@ -3,7 +3,6 @@ module github.com/hashicorp/terraform-plugin-go
 go 1.16
 
 require (
-	github.com/golang/protobuf v1.4.3
 	github.com/google/go-cmp v0.5.6
 	github.com/hashicorp/go-hclog v1.1.0
 	github.com/hashicorp/go-plugin v1.4.3