CXX-2995 bump minimum required C Driver version to d934cd5d (#1235)

* Clarify minimum C Driver version handling requirements

Author: eramongodb

.mci.yml

@@ -10,9 +10,10 @@ exec_timeout_secs: 3600
 variables:
 
     # If updating mongoc_version_minimum, also update:
-    # - the default value of --c-driver-build-ref in etc/make_release.py
     # - LIBBSON_REQUIRED_VERSION and LIBMONGOC_REQUIRED_VERSION in CMakeLists.txt
     # - the version of pkg:github/mongodb/mongo-c-driver in etc/purls.txt
+    # - the default value of --c-driver-build-ref in etc/make_release.py
+    # Only LIBMONGOC_DOWNLOAD_VERSION needs to be updated when pinning to an unreleased commit.
     mongoc_version_minimum: &mongoc_version_minimum "1.28.0"
 
     integration_matrix:

CMakeLists.txt

@@ -54,7 +54,7 @@ set(LIBBSON_REQUIRED_ABI_VERSION 1.0)
 
 # Also update etc/purls.txt.
 set(LIBMONGOC_REQUIRED_VERSION 1.28.0)
-set(LIBMONGOC_DOWNLOAD_VERSION 1.28.0)
+set(LIBMONGOC_DOWNLOAD_VERSION d934cd5de55af65220816e4fd01ce3f9c0ef1cd4) # TODO: update to 1.29.0 once it is released.
 set(LIBMONGOC_REQUIRED_ABI_VERSION 1.0)
 
 set(NEED_DOWNLOAD_C_DRIVER false)