[c++] update Boost to at least 1.86

[jameslamb]

Summary

As of this writing, LightGBM is using Boost 1.74

https://github.com/microsoft/LightGBM/blob/195c26fc7b00eb0fec252dfe841e2e66d6833954/.ci/setup.sh#L110-L112

https://github.com/microsoft/LightGBM/blob/195c26fc7b00eb0fec252dfe841e2e66d6833954/cmake/IntegratedOpenCL.cmake#L2

The project should update to at least Boost 1.86.

Motivation

• necessary to support the GPU build on Visual Studio 2022: https://github.com/microsoft/LightGBM/pull/6953#discussion_r2162325529
• makes it possible for users to build from source on newer Boost versions
  • which, for example, they may have because newer versions are the default in their Linux distribution: https://github.com/microsoft/LightGBM/issues/6786#issuecomment-2651646482
• would allow eliminating a patch in the conda-forge recipe:
  • https://github.com/conda-forge/lightgbm-feedstock/pull/69

Description

N/A

References

Some notes on previous attempts:

• https://github.com/microsoft/LightGBM/pull/6953#discussion_r2161988434
• https://github.com/microsoft/LightGBM/pull/6582