Migrate Sites to Pantheon Using Guided Migration Doc Update 

Re: [Migrate Sites to Pantheon Using Guided Migration](https://docs.pantheon.io/guides/guided/migrate)

Priority: Low

## Issue Description:

Some screenshots are outdated. Step 2 still includes Drupal 8 as an option. Currently we don't have this anymore.

## Suggested Resolution

Update this w/ the actual applicable process For D8-D11.