refactor(readme): remove deprecated Ubuntu 18.04 LTS entry

Author: dgibbs64

README.md

@@ -19,7 +19,6 @@ SteamCMD is a command-line version of the Steam client. It allows you to downloa
 - `ubuntu-24.04` - Ubuntu 24.04 LTS 'Noble Numbat'
 - `ubuntu-22.04` - Ubuntu 22.04 LTS 'Jammy Jackalope'
 - `ubuntu-20.04` - Ubuntu 20.04 LTS 'Focal Fossa'
-- `ubuntu-18.04` - Ubuntu 18.04 LTS 'Bionic Beaver'
 
 ## Usage