CXX-3087 Replace single-argument FetchContent_Populate with FetchContent_MakeAvailable

[eramongodb]

Resolves CXX-3087. Verified by this patch.

Addresses the following warning emitted by CMake 3.30 and newer for mongo-c-driver and EP_mnmlstc_core:

CMake Warning (dev) at /snap/cmake/1409/share/cmake-3.30/Modules/FetchContent.cmake:1953 (message):
  Calling FetchContent_Populate(mongo-c-driver) is deprecated, call
  FetchContent_MakeAvailable(mongo-c-driver) instead.  Policy CMP0169 can be
  set to OLD to allow FetchContent_Populate(mongo-c-driver) to be called
  directly for now, but the ability to call it with declared details will be
  removed completely in a future version.
Call Stack (most recent call first):
  cmake/FetchMongoC.cmake:24 (FetchContent_Populate)
  CMakeLists.txt:78 (include)
This warning is for project developers.  Use -Wno-dev to suppress it.

This is caused by CMP0169:

Calling FetchContent_Populate() with a single argument (the name of a declared dependency) is deprecated. [...] The OLD behavior of this policy allows FetchContent_Populate() to be called with the name of a declared dependency. The NEW behavior halts with a fatal error in such cases.

This PR replaces such calls with FetchContent_MakeAvailable() accordingly.

This was straightforward for mongo-c-driver given it was (manually) implementing add_subdirectory() behavior, which is consistent with FetchContent_MakeAvailable()'s regular behavior, which internally calls add_subdirectory() on population.

EP_mnmlstc_core required a workaround due to the SOURCE_SUBDIR option only being supported in 3.18 and newer. Using SOURCE_SUBDIR to prevents the internal add_subdirectory() call is an officially supported pattern which is needed by FetchMnmlstcCore.cmake to prevent mnmlstc/core's configurations from leaking into the C Driver's configuration (see #1019 for details). This workaround can be removed once the minimum required CMake version is raised to 3.18 or newer.

[kevinAlbs]

Suggested change

	SOURCE_SUBDIR ""
	SOURCE_SUBDIR "" # To prevent adding project to CMake.