undefined method 'window' for Selenium::Session

Upgraded to flow master to try this new stuff out, but getting compile errors:

```
In spec/support/flows/authenticated_flow.cr:15:13

15 | session.window.resize_to(2560, 1366)
^-----
Error: undefined method 'window' for Selenium::Session
```

This is related to the new selenium shard. I think the new method is `window_manager`, but it made me think that we will need to make sure we have a clear upgrade guide on this. So when we do the next release, we will need to document all of the method name changes.

/cc @matthewmcgarvey

Start with the failing call in spec/support/flows/authenticated_flow.cr and review the Selenium shard changes around Session.window and window_manager. Identify the method-name changes that affect upgrading to the next release, then document them in a clear upgrade guide. Done means the guide covers the relevant renamed methods and explains the updated usage.

Written by the indexing model from the issue text.