When using Lightning in logical mode for parallel data import, the error of 'Config: ErrInvalidConfig' should be avoided.

[Frank945946]

Bug Report

In version 7.3, the original 'on-duplicate' conflict strategy for logical imports has been replaced by 'conflict strategy.' However, 'conflict strategy' cannot be applied in parallel import scenarios. When 'parallel-import' is set to 'true' (resulting in an error [Lightning:Config:ErrInvalidConfig] 'conflict.strategy cannot be used with tikv-importer.parallel-import'). Upgrading to version 7.3 or later and using the original configuration may cause task startup failures. To address this, it is necessary to lift this restriction, ensuring compatibility with existing user behavior. Therefore, when utilizing Lightning for logical imports with 'parallel-import' set to 'true,' the use of 'conflict strategy' should be allowed without triggering errors.

1. Minimal reproduce step (Required)

When the 'backend' parameter in Lightning is set to 'tidb' and 'parallel-import' is also set to 'true,'

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

No error will be returned and the task be successfully submitted.

3. What did you see instead (Required)

An error will occur.[Lightning:Config:ErrInvalidConfig] 'conflict.strategy cannot be used with tikv-importer.parallel-import

4. What is your TiDB version? (Required)

7.5.1,7.6