Go quick start updates: fetch specific version of grpc-go repo, update protoc-gen-go URL

[chalin]

• Once (if) Go quick start: use protoc-gen-go-grpc@v1.0 #436 is merged and the next release (>1.32.0) of grpc-go comes out, then we'll need to undo the changes, shown below, that were introduced by Go quick start: use protoc-gen-go-grpc@v1.0 #436. These changes will ensure that we have readers download a specific version of the grpc-go repo - Fetch specific version of grpc-go repo in quick start and tutorial pages #504
• From Go quick start: use protoc-gen-go-grpc@v1.0 #436 (comment): also update the command to get protoc-gen-go - Go quick start: use protoc-gen-go from new package #475

---

    ```sh
-   $ git clone -b {{< param grpc_go_release_tag >}} https://github.com/grpc/grpc-go
+   $ git clone https://github.com/grpc/grpc-go
    ```

  [Basics tutorial]: ../basics/
- [download]: https://github.com/grpc/grpc-go/archive/{{< param grpc_go_release_tag >}}.zip
+ [download]: https://github.com/grpc/grpc-go/archive/master.zip
  [Getting Started]: https://golang.org/doc/install