Upgrade CMake minimum version

[gaotongsh]

Description

Right now this project requires a minimum CMake version of 3.2. Since the new release of CMake (3.27), versions < 3.5 are considered deprecated and will be removed in the future.

This is why we should increase the CMake minimum version. A possible new minimum version is 3.16 as this is the one that ships with Ubuntu 20.04.

An issue of the same nature can be found in other projects like nlohmann/json#4076.

How to reproduce

Build with CMake v3.28.1.

cmake ..
CMake Deprecation Warning at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value or use a ...<max> suffix to tell
  CMake that the project does not need compatibility with older versions.

Checklist

IMPORTANT: We will close issues where the checklist has not been completed.

Please provide the following information:

• librdkafka version (release number or git tag): v2.3.0
• Operating system: Ubuntu 20.04.5