Drop 18.04 from versions to be shown on homepage

as it has fallen out of support

source/conf-common.py

@@ -16,7 +16,7 @@ import sys, os, datetime
 
 html_context = {
   #mahara_releases is used to build the user manual links
-  'mahara_releases': ['19.10', '19.04', '18.10', '18.04'],
+  'mahara_releases': ['19.10', '19.04', '18.10'],
   #all_mahara_releases is used to build the full list of user manual links on all_releases.html
   'all_mahara_releases': ['19.10', '19.04', '18.10', '18.04', '17.10', '17.04', '16.10', '16.04', '15.10', '15.04', '1.10', '1.9', '1.8', '1.7', '1.6', '1.5', '1.4']
 }