docs: Update BUILD.md for Conan 2 (#5478)

This change updates BUILD.md for Conan 2, add fixes/workarounds for Apple Clang 17, Clang 20 and CMake 4. This also removes (from BUILD.md only) workarounds for compiler versions which we no longer support e.g. Clang 15 and adds compilation flag -Wno-deprecated-declarations to enable building with Clang 20 on Linux.

Author: Bronek