Merge pull request rails#33929 from yahonda/update_utf8mb4_railsguide

Update MySQL server version and character set in Rails Guide [skip ci]

Author: kamipo

guides/source/configuring.md

@@ -1014,7 +1014,6 @@ If you choose to use MySQL or MariaDB instead of the shipped SQLite3 database, y
 ```yaml
 development:
   adapter: mysql2
-  encoding: utf8
   database: blog_development
   pool: 5
   username: root
@@ -1024,6 +1023,8 @@ development:
 
 If your development database has a root user with an empty password, this configuration should work for you. Otherwise, change the username and password in the `development` section as appropriate.
 
+NOTE: If your MySQL version is 5.5 or 5.6 and want to use the `utf8mb4` character set by default, please configure your MySQL server to support the longer key prefix by enabling `innodb_large_prefix` system variable.
+
 Advisory Locks are enabled by default on MySQL and are used to make database migrations concurrent safe. You can disable advisory locks by setting `advisory_locks` to `false`:
 
 ```yaml