Dependency updates

• abseil-cpp 20200225.2 -> 20200923
• Protobuf 3.12.2 -> 3.13.0

Platform changes

• Added support for Python3.9 (#2187)
• Dropped support for Python3.5 (#2186)
• Added support for Ubuntu 20.10 (#2188)
• Dropped support for Ubuntu 16.04 LTS and Ubuntu 19.10 (#2188)

Known Breaking Change

• Since last synchronization with Google internal code base, now Routing and SAT (i.e. OR-Tools) use some C++17.
  WARNING: If you provide your own version of abseil-cpp please verify it is built against C++17 too.
• The MPSolver::CreateSolver signature has been changed. The model name argument has been dropped.

CMake

• Fix disabling SCIP support when using -DUSE_SCIP=OFF (#2129)
• Integrate samples and examples to the CMake build system.
  note: can be disable by using -DBUILD_SAMPLES=OFF and -DBUILD_EXAMPLES=OFF.
  note: can be disable for a specific language using -DBUILD_<LANG>_SAMPLES=OFF or
  -DBUILD_<LANG>_EXAMPLES=OFF (with <LANG> among CXX, PYTHON, JAVA and DOTNET).

Make

• Require Make >= 4.3 (use of Make eval function).
• Require CMake >= 3.14 (use of CMake --verbose option).
• Add option to disable SCIP support using USE_SCIP=OFF (#2134)
• Add option to disable CLP and CBC support using USE_COINOR=OFF

Java

• OR-Tools now generate maven packages (#202).

Bug fixes

• Fix C++ and Python build on FreeBSD (#2126).
• Fix build in debug on windows (#2077).
• Fix long standing crash in parallel on CP-SAT on Windows (#2001, #2019).