Reduce amount of CMake warnings in MITK

Summary:

• Use Boost_ROOT instead of BOOST_ROOT
• Use DOWNLOAD_EXTRACT_TIMESTAMP option in ExternalProject_Add() via policy CMP0135
• Use same minimum required CMake version in CppMicroServices as in MITK

Fixes T30216

Test Plan: Manually built with a recent version of CMake. CI for checking older versions.

Reviewers: floca, O1 MITK Reviewer Group I

Reviewed By: floca, O1 MITK Reviewer Group I

Maniphest Tasks: T30216

Differential Revision: https://phabricator.mitk.org/D867