Update development environment setup guide

[marcogschmidt]

Describe the requested changes Development environment setup guide is out of date. We should update the doc and also the BUILD.md file.

Link to any relevant existing docs

1. https://docs.solo.io/gloo/latest/guides/dev/setting-up-dev-environment/

Additional context Add any other context about the problem here.

[corporealshift]

Here are a few things I noticed right away:

• mockgen is a dependency but it isn't mentioned as one.
• dep is no longer required