Update outdated C++17 and Python 2 references (#5045)

Summary:
- Update C++ language level from C++17 to C++20 in `CONTRIBUTING.md` to match the actual CMake configuration (`CMAKE_CXX_STANDARD 20` in the root `CMakeLists.txt`)
- Remove outdated "progressively dropping python2 support" note from `contrib/README.md` (Python 2 reached end-of-life in January 2020 and Faiss requires Python 3)
- Update shebangs from `python2` to `python3` in three benchmark scripts: `benchs/kmeans_mnist.py`, `benchs/bench_gpu_1bn.py`, and `benchs/bench_vector_ops.py`

Pull Request resolved: https://github.com/facebookresearch/faiss/pull/5045

Test Plan:
- [ ] No functional code changes; only documentation and shebangs are modified
- [ ] Verified C++20 is the actual standard by checking `CMakeLists.txt` (`set(CMAKE_CXX_STANDARD 20)`) and `INSTALL.md` (which already references "a C++20 compiler")

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed By: mnorris11

Differential Revision: D99691731

Pulled By: alibeklfc

fbshipit-source-id: 579cf21054cdf6bdaea27f5abb1c56e0b709a922

CONTRIBUTING.md

@@ -44,7 +44,7 @@ outlined on that page and do not file a public issue.
 
 * 4 spaces for indentation in C++ (no tabs)
 * 80 character line length (both for C++ and Python)
-* C++ language level: C++17
+* C++ language level: C++20
 
 ## License
 

benchs/bench_gpu_1bn.py

@@ -1,4 +1,4 @@
-#! /usr/bin/env python2
+#! /usr/bin/env python3
 # Copyright (c) Meta Platforms, Inc. and affiliates.
 #
 # This source code is licensed under the MIT license found in the

benchs/bench_vector_ops.py

@@ -1,4 +1,4 @@
-#! /usr/bin/env python2
+#! /usr/bin/env python3
 # Copyright (c) Meta Platforms, Inc. and affiliates.
 #
 # This source code is licensed under the MIT license found in the

benchs/kmeans_mnist.py

@@ -1,4 +1,4 @@
-#! /usr/bin/env python2
+#! /usr/bin/env python3
 # Copyright (c) Meta Platforms, Inc. and affiliates.
 #
 # This source code is licensed under the MIT license found in the

contrib/README.md

@@ -8,8 +8,6 @@ The contrib directory contains helper modules for Faiss for various tasks.
 The contrib directory gets compiled in the module faiss.contrib.
 Note that although some of the modules may depend on additional modules (eg. GPU Faiss, pytorch, hdf5), they are not necessarily compiled in to avoid adding dependencies. It is the user's responsibility to provide them.
 
-In contrib, we are progressively dropping python2 support.
-
 ## List of contrib modules
 
 ### rpc.py