Updating Milvus2 implementation for new dependency management

[MichelBartels]

Proposed changes:
The new dependency management was not working properly with Milvus2 (reference to milvus2 install option, but not present in setup.cfg and __init__.py in document_stores did not load Milvus2DocumentStore correctly). This PR fixes this.

Status (please check what you already did):

• First draft (up for discussions & feedback)
• Final code
• Added tests
• Updated documentation

[ZanSara]

I'm not sure we want to have 3 separate extras for Milvus though. Is there a special reason to pin the release candidate 6 in a separate one? I definitely made a mistake by leaving references to milvus2 around, but the aim was to have only milvus1 (<2.0.0) and milvus (>=2.0.0), as a sign that soon Milvus 2 will soon be our default. However, aggregate extras like docstores will still use milvus1 until the code enabled by milvus becomes stable.

Let me know if I missed something though! Maybe we really want to keep the rc6 into our dependencies for some reason.

[MichelBartels]

You are right. I was a confused by the reference to "milvus2".
Then, however, we should also change "milvus" in this line to "milvus1".

The current implementation of Milvus2DocumentStore does require rc6. However, I have already been working on updating this document store as part of #2120.
I think I'm going to close this PR for now and separate the doc store update of #2120 into a separate PR which will also fix the references to the Milvus extras.