Add BRAVIA 4K VH21 to known devices

[jayofdoom]

I got the error:

INFO (zeroconf-ServiceBrowser-_googlecast._tcp-1640704) [homeassistant.components.cast.helpers] Fetched cast details for unknown model 'BRAVIA 4K VH21' manufacturer: 'Sony', type: 'cast'. Please create a bug report at https://github.com/home-assistant/core/issues?q=is%3Aopen+is%3Aissue+label%3A%22integration%3A+cast%22

Following the pattern of https://github.com/home-assistant-libs/pychromecast/pull/878 and the associated issue, I figured I'd just fix it. Let me know if there are any needed modifications.