Deprecate support for directly accessing logger

[vyasr]

Description

Contributes to https://github.com/rapidsai/build-planning/issues/104

This PR removes support for accessing rmm's underlying spdlog logger directly.

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.

[wence-]

Setting to do not merge, I think the Cython bindings in rmm/_lib/logger.pyx also want updated.