add master symlink to prevent legacy from showing up on master

Author: lindseymoore

config/redirects

@@ -3,6 +3,7 @@ define: base https://www.mongodb.com/${prefix}
 define: versions v1.7 v1.8 v1.9 v1.10 v1.11 v1.12 v1.13 v1.14 v1.15 v1.16 v1.17 v2.0 v2.1 master
 
 symlink: current -> master
+symlink: upcoming -> master
 
 raw: ${prefix}/ -> ${base}/current/
 raw: ${prefix}/stable -> ${base}/current/