Upgrade OR-Tools to C++20 across all platforms

[gchatelet]

• source: cleanup code (80%)
• bazel: migrate to c++20
  • bazel: update bazel/README.md
• cmake: migrate UNIX platforms to C++20

[Mizux]

missing README.md

or-tools/bazel/README.md

Lines 77 to 81 in af5487d

	* on UNIX:
	```sh
	bazel build -c opt --cxxopt=-std=c++17 ...
	```

and

or-tools/bazel/README.md

Lines 93 to 97 in af5487d

	* on UNIX:
	```sh
	bazel test -c opt --cxxopt=-std=c++17 ...
	```

note: Since now, we have a bazelrc, this is not needed anymore ? (and both windows and unix can use the same cmd without any option)

[Mizux]

also spotted:

or-tools/ortools/algorithms/binary_search.h

Lines 191 to 192 in af5487d

	// For integers using two's complement (all compilers in practice up to
	// c++17, and all compilers in theory starting from c++20), we can use a

or-tools/ortools/base/filesystem.cc

Line 18 in af5487d

#include <filesystem> // NOLINT(build/c++17)

or-tools/ortools/sat/colab/README.md

Line 15 in af5487d

bazel run -c opt --cxxopt=-std=c++17 ortools/python:ortools_notebook

or-tools/ortools/sat/colab/README.md

Line 23 in af5487d

bazel run -c opt --cxxopt=-std=c++17 ortools/python:ortools_notebook -- --no-browser

or-tools/ortools/third_party_solvers/xpress_environment.cc

Line 19 in af5487d

// NOLINTNEXTLINE(build/c++17)

[gchatelet]

I think it's all fixed, let me know if you see anything else.

[Mizux]

LGTM