azure-storage-blob 12.0.0

### Which service(blob, file, queue) does this issue concern?
Blob

### Which version of the SDK was used? Please provide the output of `pip freeze`.
azure-storage-blob 12.0.0

### What problem was encountered?
cannot import name 'BlockBlobService' from 'azure.storage.blob'

### Have you found a mitigation/solution?
Revert back to v2.1.0

I just want to add that in [pypi](https://pypi.org/project/azure-storage-blob/) the documentation mentions to use `from azure.storage.blob import BlobServiceClient`. However the documentation found in [https://azure-storage.readthedocs.io/index.html](https://azure-storage.readthedocs.io/index.html) still mentions using BlockBlobService instead. So can I clarify which package are we expected to import going forward?