Validate if `utf8mb4` character set and longer index key prefix is supported

Once #33608 merged If users create a new database using MySQL 5.1.x, it will fail to create databases
since MySQL 5.1 does not know `utf8mb4` character set.

This pull request removes `encoding: utf8mb4` from `mysql.yml.tt`
to let create_database method handles default character set by MySQL server version.

`supports_longer_index_key_prefix?` method will need to validate if MySQL 5.5 and 5.6 server configured
correctly to support longer index key prefix, but not yet.

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -114,6 +114,14 @@ def supports_advisory_locks?
         true
       end
 
+      def supports_longer_index_key_prefix?
+        if mariadb?
+          version >= "10.2.2"
+        else
+          version >= "5.7.9"
+        end
+      end
+
       def get_advisory_lock(lock_name, timeout = 0) # :nodoc:
         query_value("SELECT GET_LOCK(#{quote(lock_name.to_s)}, #{timeout})") == 1
       end
@@ -250,8 +258,12 @@ def recreate_database(name, options = {})
       def create_database(name, options = {})
         if options[:collation]
           execute "CREATE DATABASE #{quote_table_name(name)} DEFAULT COLLATE #{quote_table_name(options[:collation])}"
+        elsif options[:charset]
+          execute "CREATE DATABASE #{quote_table_name(name)} DEFAULT CHARACTER SET #{quote_table_name(options[:charset])}"
+        elsif supports_longer_index_key_prefix?
+          execute "CREATE DATABASE #{quote_table_name(name)} DEFAULT CHARACTER SET `utf8mb4`"
         else
-          execute "CREATE DATABASE #{quote_table_name(name)} DEFAULT CHARACTER SET #{quote_table_name(options[:charset] || 'utf8mb4')}"
+          execute "CREATE DATABASE #{quote_table_name(name)} DEFAULT CHARACTER SET `utf8`"
         end
       end
 

railties/lib/rails/generators/rails/app/templates/config/databases/mysql.yml.tt

@@ -11,7 +11,6 @@
 #
 default: &default
   adapter: mysql2
-  encoding: utf8mb4
   pool: <%%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
   username: root
   password: