some dm prechecks are too strict to users and may block them creating a task

[lichunzhu]

Is your feature request related to a problem?

Current DM precheck has too many must-pass items and non-must-pass items. Refer to the document https://docs.pingcap.com/zh/tidb/stable/dm-precheck#%E4%BB%BB%E5%8A%A1%E5%89%8D%E7%BD%AE%E6%A3%80%E6%9F%A5

And too many must-pass items will greatly reduce the success rate of user configuration tasks, so you need to change some of the must-pass items to non-must-pass items. Non-must-pass items alert the user if there are abnormal check results, and after the user confirms these alerts, the task can continue to run without blocking the user.

Some of the must-pass items are listed below, but the actual items can be changed to non-must-pass items.

1. (Required) Check if there are compatibility differences in character sets

    It is recommended to change it to a non-must-pass item, because the business constraints, the user will not be able to go upstream to modify the character set for a while. If the case is encountered, Warning prompts the user to convert to the target table table by utf8mb4 by default. Do not block the user.
   

2. (Required) Check if primary key or unique key constraint exists in upstream table (introduced from v1.0.7)

    It is recommended to change to a non-mandatory item, because it is often encountered that the user's existing business table does not have a primary key, the business can not directly add a primary key, if the case is encountered, the Warning prompts the user no primary key table data synchronization performance will be relatively slow, and at the same time, after an abnormal retry, the target table may be more than the original table of duplicate records.
   

3. (Required) Database master-slave configuration, the upstream database must set the database ID server_id (non-AWS Aurora environment is recommended to enable GTID)

    It is recommended to change it to a non-mandatory item, because the modification requires restarting the DB, and the business generally does not easily accept the restart, so it is recommended to change it to a non-mandatory item.    
   

4. (Required) Check whether the upstream is in the Online-DDL process.

    It is recommended to change to non-mandatory.
   

Describe the feature you'd like

Cut these four precheck loose.

Describe alternatives you've considered

No response

Teachability, Documentation, Adoption, Migration Strategy

No response