Convert charset should consider index key length limitation

[tangenta]

Bug Report

Please answer these questions before submitting your issue. Thanks!

CREATE TABLE posts (
  id INT AUTO_INCREMENT PRIMARY KEY,
  title VARCHAR(500) CHARACTER SET utf8mb4,
  subtitle VARCHAR(500) CHARACTER SET utf8mb4,
  UNIQUE KEY (title, subtitle)); 

ERROR 1071 (42000): Specified key was too long (4000 bytes); max key length is 3072 bytes

1. Minimal reproduce step (Required)

CREATE TABLE posts (
  id INT AUTO_INCREMENT PRIMARY KEY,
  title VARCHAR(500) CHARACTER SET utf8,
  subtitle VARCHAR(500) CHARACTER SET utf8,
  UNIQUE KEY (title, subtitle)); 

ALTER TABLE posts CONVERT TO CHARACTER SET utf8mb4;

2. What did you expect to see? (Required)

ERROR 1071 (42000): Specified key was too long (4000 bytes); max key length is 3072 bytes

3. What did you see instead (Required)

Query OK, 0 rows affected (0.07 sec)

4. What is your TiDB version? (Required)

cc37099

[fzzf678]

/assign