refactor!: Qdrant - remove `index` parameter from methods

[anakin87]

Some methods of Qdrant Document Store allow specifying the index parameter.

This comes from the Haystack 1.x implementation. In 2.x, we are enforcing the convention where a Document Store corresponds to a specific index/collection.

Having some methods where you can specify index is inconsistent and fuels user confusion. See https://github.com/deepset-ai/haystack-core-integrations/issues/942

Proposed Changes:

• remove index parameter from internal and public methods
• this is a breaking change -> I will release a new major version

How did you test it?

CI

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
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