Update RED section - EOL RED products in description

https://docs.sophos.com/nsg/sophos-firewall/20.0/Help/en-us/webhelp/onlinehelp/AdministratorHelp/SystemServices/RED/index.html

Note

The RED unified firmware option is only available on RED15(w) and RED50 devices.

RED15 and RED50 are EOL devices. This should either be replaced with RED20 and RED60 or RED20 and RED60 should be added to the text, as they don't support the legacy firmware.