Replace libfmt with std::format

[nyllet]

Issue

This is an alternative solution to https://github.com/Project-OSRM/osrm-backend/issues/7238

Tasklist

• [ x] CHANGELOG.md entry (How to write a changelog entry)
• [ ] update relevant Wiki pages
• [ ] add tests (see testing documentation)
• [ ] review
• [ ] adjust for comments

Requirements / Relations

none

[afarber]

Please do not forget to remove the fmt from scripts/update_dependencies.sh

[afarber]

Hi @nyllet, your PR removes the fmt library entirely and switches directly to std::format...

However, this might cause build failures on systems where std::format is incomplete. For example:

• Clang with older libstdc++: The <format> header exists, but symbols are missing at link time
• Some GCC 13 installations: Have incomplete chrono formatting support
• Alpine Linux with GCC 14: Missing chrono formatters

I've just submitted #7261 which takes a different approach:

• Adds a thin compatibility layer (util/format.hpp) that automatically uses std::format when available
• Falls back to fmt::format on systems where std::format doesn't work
• Uses CMake runtime detection (check_cxx_source_runs) to test compile, link and execution
• Also fixes the node_osrm C++17/C++20 ABI mismatch that was overlooked in #6877

This way, we can:

1. Keep fmt as a fallback for systems with incomplete std::format
2. Gradually phase out fmt as older compilers are retired
3. Avoid breaking builds for users on platforms with partial C++20 support

Would you be open to reviewing #7261 as an alternative approach?