Update min required version to 3.6

Author: aljones15

lib/authn.js

@@ -156,15 +156,15 @@ function _checkServerVersion({serverInfo, config}) {
     url: urls.sanitize(config.url),
     version,
   });
-  // if the server is not at least 2.6.0 throw
-  // as we do require roles / authn
-  if(!semver.gte(version, '2.6.0')) {
+  // if the server is not at least 3.6.0 throw
+  // as mongo node driver 4 only supports 3.6.0 and higher
+  if(!semver.gte(version, '3.6.0')) {
     throw new BedrockError(
       'Unsupported database version.',
       'NotSupportedError', {
         url: urls.sanitize(config.url),
         version,
-        required: '>= 2.6.0'
+        required: '>= 3.6.0'
       });
   }
   if(config.requirements.serverVersion &&