Update BUILD for conan2, clang-20 compatibility fixes

[Bronek]

High Level Overview of Change

Update BUILD.md for Conan 2, add fixes/workarounds for Apple Clang 17, Clang 20 and CMake 4

Context of Change

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.

Type of Change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Refactor (non-breaking change that only restructures code)
• [ ] Performance (increase or change in throughput and/or latency)
• [ ] Tests (you added tests for code that already exists, or your new feature included in this PR)
• [x] Documentation update
• [ ] Chore (no impact to binary, e.g. .gitignore, formatting, dropping support for older tooling)
• [ ] Release

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 78.8%. Comparing base (4eae037) to head (11862bd). :warning: Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #5478     +/-   ##
=========================================
- Coverage     78.8%   78.8%   -0.0%     
=========================================
  Files          814     814             
  Lines        71283   71282      -1     
  Branches      8349    8366     +17     
=========================================
- Hits         56172   56152     -20     
- Misses       15111   15130     +19     

see 7 files with indirect coverage changes

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[bthomee]

@Bronek ready to merge?