[2.0] Deprecations to drop

[olleolleolle]

This PR is a "quest issue" about removing deprecated functionality in the next major version.

Description

We have some already-described deprecations, and can now remove them.

If you find more deprecated functionality, please add it to this list, which is meant to guide the 2.0 release.

• FilePart: Drop the "consistency between UploadIO" method

  faraday/lib/faraday/file_part.rb

  Line 57 in 21ac595

  # @deprecated Use FilePart instead of this class. It behaves identically, with

• Remove deprecated Faraday::Request#method #1303: Drop the method alias for http_method.
• Drop deprecated auth helpers from Connection and refactor auth middleware #1308: Authorization: drop the helpers available in Faraday::Connection