fix: migrator for mysql 5.6

[black-06]

• Do only one thing
• Non breaking API changes
• Tested

What did this pull request do?

fix migrator for mysql 5.6.

Add DontSupportDropConstraint because MySQL 5.x does not support drop constraint for any type.

And it has been tested here at go-gorm/gorm#6822 with commit go-gorm/gorm@288c065

User Case Description

[black-06]

@a631807682 But I wonder why this test has not failed before, It has a history of 3 years

https://github.com/go-gorm/gorm/blob/9514d5f9e677e0c94d3d4e48d4f08c649f092d9a/tests/migrate_test.go#L669C1-L693C2

the error should be:

Error 1064 (42000): You have an error in your SQL syntax; 
check the manual that corresponds to your MySQL server version for the right syntax 
to use near 'CONSTRAINT `fk_users_account`' at line 1

[a631807682]

@a631807682 But I wonder why this test has not failed before, It has a history of 3 years

go-gorm/gorm@9514d5f/tests/migrate_test.go#L669C1-L693C2

the error should be:

Error 1064 (42000): You have an error in your SQL syntax; 
check the manual that corresponds to your MySQL server version for the right syntax 
to use near 'CONSTRAINT `fk_users_account`' at line 1

e05761b#diff-a5ca21254ced8196f81a4c4e926af32db03cb0ea78a1f853835300f6dda3f0c6L212

[black-06]

e05761b#diff-a5ca21254ced8196f81a4c4e926af32db03cb0ea78a1f853835300f6dda3f0c6L212

Stupid me....😭