Motivation

On evergreen, the two active C++ driver projects (development and release) build against two versions of the C driver:

• The tip of the C driver development branch (most tasks)
• The minimum required version of the C driver (on the Ubuntu 18.04 with minimum libmongoc variant)

There are some points of friction:

• On the C++ release branch being tested, if C later deprecates API, the release branch will fail with deprecation warnings. It is atypical to backport a change to remove the deprecated C API.
• C driver behavior changes may cause test failures and interfere with development. For example, CDRIVER-3958 changed the output of JSON, but did not necessitate a bump in the C++ driver's minimum dependency.

Scope
TBD. Investigate and decide on a strategy for what versions of the C driver makes sense to test against, and what the process should be for handling C behavior changes and deprecations.