Unifying target version parameter for `upgrade_document`

[tom-tan]

The current implementation of upgrade_document has the following signature:

def upgrade_document(
    document: Any, v1_only: bool, v1_1_only: bool, output_dir: str, imports: Set[str]
) -> Any:

IMO it is not scalable for future releases because the new parameter should be added for each release version.

How about fixing the signature as follows?

def upgrade_document(
    document: Any, target_version: Optional[str] = "latest", output_dir: str, imports: Set[str]
) -> Any:

The target_version parameter takes a version string such as "v1.0", "v1.1", "v1.2" and "latest" (same as "v1.2").
It enables upgrading upgrade_document to other future release versions without changing its signature.

If it is OK for this change, I will send a pull request for it.

[mr-c]

Great idea, please do so