www/osrm-backend: update to 6.0.0.g20250916

• fixes the build with Boost 1.87 and later from upstream
• port has also been updated to the latest commit

Other changes:

• upstream has decided to remove their header files and libraries from their packages, leaving them as an implementation detail.
• various fixes to speed limits.
• various fixes to unit tests.
• remove a Linux-only tool from the CMakeLists.txt file; it was never installed and would fail on run-time due to it not being implemented for FreeBSD.

Changelog: https://github.com/Project-OSRM/osrm-backend/releases/tag/v6.0.0

PR: 285105
Event: EuroBSDcon Devsummit 2025