Switched to C++20 and dropped the dependency on the external FMT code

added Dependencies GeoModelIO labels

requested review from @rbianchi

@rbianchi, are you OK with all this?

resolved all threads

Maybe can update (not in this MR?) the cmake_minimum_required(VERSION 3.16...3.26) next; the max version could go to 3.29 now (or higher?)

added 4 commits

• a185a763...6183ebfb - 2 commits from branch main
• 5c3df51b - Switched to C++20
• cb144d59 - Switched from `fmt::format` to `std::format`

Compare with previous version

added 1 commit

• bf0df30b - Do cmake_minimum_required centrally in one single place

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resolved all threads

merged

Just fyi, this removes compatibility with GCC11.

Yes, that's true, you're right. I also have problems building GeoModel on my laptop, which still runs macOS 13.

However, at one of the latest weekly meetings, it was decided to push for full C++20 and discontinue support for older platforms, and encourage users to update their compilers.

(In particular, as a side note, for macOS, that means that GeoModel can now be built natively only on the two most recent versions, currently Sonoma and Sequoia)