Drop support for Ubuntu 18.04

Co-authored-by: Tony Afanasev <anton.afanasev@xiag.ch>

Author: sonicdoe

.github/workflows/main.yml

@@ -21,7 +21,6 @@ jobs:
           - debian:11
           - debian:10
           - ubuntu:20.04
-          - ubuntu:18.04
         node-version:
           - 16
           - 14

README.md

@@ -33,7 +33,7 @@ detect-character-encoding may return `null` if no charset matches.
 ## Supported operating systems
 
 - macOS Big Sur
-- Ubuntu 20.04 and 18.04
+- Ubuntu 20.04
 - Debian 12, 11, and 10
 
 detect-character-encoding does not support 32-bit operating systems.