Replace Connection class with xcc.Connection

[Mandrenkov]

Context: It was decided that the strawberryfields.api.Connection class should be replaced by the xcc.Connection class.

Description of the Change:

• [Main] Deleted the strawberryfields.api.Connection class and its corresponding unit tests.
• [Main] Updated RemoteEngine and the remaining API code to use the xcc.Connection class.
• [Main] Fixed, refactored, and enabled the tests in tests/api/test_remote_engine.py.
• [Main] Added a "Connection" section to the v0.20.0 migration guide.
• [Secondary] Sorted the installation requirements (and added XCC as a dependency).
• [Secondary] Added a help entry for the docs Makefile target.

Benefits:

• Strawberry Fields can leverage the Xanadu Cloud 0.4.0 API.
• There is less API code that needs to be maintained.

Possible Drawbacks:

• Job results stored in the .npy format cannot be retrieved.
• Existing code which directly instantiates a strawberryfields.api.Connection object will need to be updated.

Related GitHub Issues: None.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (integrate-xanadu-cloud-client@fda8956). Click here to learn what that means. The diff coverage is n/a.

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