MDEV-19123 Change default charset from latin1 to utf8mb4

Changing the default server character set from latin1 to utf8mb4.

mysql-test/main/innodb_ext_key.test

@@ -232,7 +232,7 @@ set optimizer_switch=@save_optimizer_switch;
 
 CREATE TABLE t1 (
   a int NOT NULL, b varchar(1) NOT NULL, KEY(a), KEY(b,a)
-) ENGINE=MyISAM;
+) ENGINE=MyISAM CHARSET=latin1;
 INSERT INTO t1 VALUES
   (0,'j'), (8,'v'), (1,'c'), (8,'m'), (9,'d'), 
   (24,'d'), (6,'y'), (1,'t'), (6,'d'), (2,'s');
@@ -653,7 +653,7 @@ CREATE TABLE t2 (
   a VARCHAR(20),
   KEY k1(a),
   PRIMARY KEY(pk)
-)ENGINE=INNODB;
+)ENGINE=INNODB CHARSET=latin1;
 
 INSERT INTO t2 SELECT a,a FROM t1;
 --replace_column 9 #