Update to oldest still-supported Ubuntu LTS

Author: TheAssassin

.github/workflows/main.yml

@@ -5,7 +5,7 @@ on: [push, pull_request]
 jobs:
   appimage-x86_64:
     name: AppImage x86_64
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-18.04
     env:
       ARCH: x86_64
       DIST: xenial
@@ -23,7 +23,7 @@ jobs:
 
   appimage-i386:
     name: AppImage i386
-    runs-on: ubuntu-16.04
+    runs-on: ubuntu-18.04
     env:
       ARCH: i386
       DIST: xenial