Fix connection error message regexp for Mysql2

In CI, sometimes the Mysql2 error message for a lost connection doesn't
include "MySQL server", it says "Lost connection to server".
This commit makes the "MySQL" part of the regexp optional.

Co-authored-by: Nikita Vasilevsky <nikita.vasilevsky@shopify.com>

lib/semian/mysql2.rb

@@ -22,8 +22,8 @@ module Mysql2
     include Semian::Adapter
 
     CONNECTION_ERROR = Regexp.union(
-      /Can't connect to MySQL server on/i,
-      /Lost connection to MySQL server/i,
+      /Can't connect to (?:MySQL )?server on/i,
+      /Lost connection to (?:MySQL )?server/i,
       /MySQL server has gone away/i,
       /Too many connections/i,
       /closed MySQL connection/i,