Set charset name utf8mb4 by default (#1157)

mysql2 gem dropped testing for MySQL 5.1 which was released in Dec 2008
and already EOL in Dec 2013 at efa47a9.

If we no longer support MySQL 5.1, utf8 (utf8mb3) is not appropriate
default encoding for MySQL 5.5 or higher, utf8mb4 should be set for
that.

FYI, Rails 6.0 no longer support MySQL 5.1 and set utf8mb4 by default.

rails/rails#33608
rails/rails#33853

README.md

@@ -273,7 +273,7 @@ Mysql2::Client.new(
   :database,
   :socket = '/path/to/mysql.sock',
   :flags = REMEMBER_OPTIONS | LONG_PASSWORD | LONG_FLAG | TRANSACTIONS | PROTOCOL_41 | SECURE_CONNECTION | MULTI_STATEMENTS,
-  :encoding = 'utf8',
+  :encoding = 'utf8mb4',
   :read_timeout = seconds,
   :write_timeout = seconds,
   :connect_timeout = seconds,
@@ -367,7 +367,7 @@ Use the value `mysql2` as the adapter name. For example:
 ``` yaml
 development:
   adapter: mysql2
-  encoding: utf8
+  encoding: utf8mb4
   database: my_db_name
   username: root
   password: my_password

lib/mysql2/client.rb

@@ -45,8 +45,8 @@ def initialize(opts = {})
         end
       end
 
-      # force the encoding to utf8
-      self.charset_name = opts[:encoding] || 'utf8'
+      # force the encoding to utf8mb4
+      self.charset_name = opts[:encoding] || 'utf8mb4'
 
       mode = parse_ssl_mode(opts[:ssl_mode]) if opts[:ssl_mode]
       if (mode == SSL_MODE_VERIFY_CA || mode == SSL_MODE_VERIFY_IDENTITY) && !opts[:sslca]