Migrating from Gun 2.2 to 2.3

Gun 2.3 updates Cowlib to the most recent version.

Gun 2.3 requires Erlang/OTP 24.0 or greater.

Features added

• Update Cowlib to 2.16.1.

Bugs fixed

• The documentation for the gun_cookies backend interface now explicitly states that cookie fields must be valid according to RFC6265.