Platform changes

• Ubuntu 16.04 LTS is deprecated (actions/virtual-environments#3287).
• Add support for Debian 11 (Bullseye).
• Add support for Fedora 34.
• python: use manylinux2014 image (PEP 599).
• python: add support for aarch64 linux using manylinux2014_aarch64 image.
• .Net: add .Net5 support.

Dependencies Update

• abseil-cpp 20210324.1 -> 20210324.2.
• Protobuf 3.15.8 -> 3.18.0.
• SCIP 7.0.1 -> master.
• Googletest 1.8.0 -> 1.10.0.
• python: use of warning in cp_model.py (#2530).
• python: absl-py 0.11 -> 0.13.

CMake

• Bump minimum version required 3.14 -> 3.15 (#2528).
• python: bump minimum required version 3.14 -> 3.18 (#2774).

Make

• Make based build is deprecated please migrate to Bazel or CMake

Java

• Improve robustness of the native library loader (#2742).
• Fix jvm GC crash when routing model or the constraint solver were disposed (#2091, #2466).
• Fix CP-SAT logging callback crash when using multiple workers (#2775).

CP-SAT

• Improve robustness of the LNS code (see #2525).
• Improve scheduling code: new factory methods to create fixed size intervals,
  new search heuristics, improved presolve and new linear cuts.
• Improve routing code: new dedicated LNS.
• Improve model checker. It is now more pedantic, especially w.r.t. potential overflows.
• Improve MIP code: better presolve and multiple improvements to the linear relaxation of MIP
  and CP models.
• Improve search diversity. When using more than 12 workers, add workers dedicated to
  improving the lower bound of the objective.
• Change to the parallelism code: by default, the solver will now use all available cores.
  Use the num_search_parameters to specify the level of parallelism.
• Deprecate SearchAllSolutions and SolveWithSolutionCallback.
• Python API: more pedantic checks when using var == ... or var != ... outside a model.Add() call.