treewide: Bump C++ standard version to C++23

[xokdvium]

Motivation

Compilers in nixpkgs have caught up and major distros should also have recent enough compilers. It would be nice to have newer features like more full featured ranges and deducing this.

Context

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[Mic92]

Feature list: https://en.wikipedia.org/wiki/C%2B%2B23

[Mic92]

What minimum clang/gcc versions would this mean?

[xokdvium]

What minimum clang/gcc versions would this mean?

This depends on the features we'd want to use. deducing this is available since Clang 19 and GCC 14 (what we already have in 25.05), so I guess it's pretty safe to start requiring this.

[Mic92]

Linux Distributions with Clang 19 and GCC 14 in Main Repos

• Arch Linux - Both Clang 19 and GCC 14 in [extra]/[core] repos
• openSUSE Tumbleweed (rolling) - GCC 14 and Clang 19 in main repos
• openSUSE Leap 15.6 (stable) - GCC 7 default, GCC 8-14 available (Development Tools Module)
• Fedora 40/41 - GCC 14 in main repo (Clang 19 requires LLVM repo)
• Ubuntu 25.04 (non-LTS) - GCC 14 default, LLVM 20 in main (Clang 19 via LLVM repo)
• Ubuntu 24.04 (LTS) - Neither in main repo (requires external repos)
• Debian Sid (unstable) - GCC 14 in main repo (Clang 19 requires LLVM repo)
• Debian 12 (Bookworm/stable) - GCC 12 and Clang 14 in main repo
• FreeBSD 14 - Clang 16/17 in base, GCC 14 and Clang 19 available in ports (lang/gcc14, devel/llvm19)

[Mic92]

So looks like distributions should be covered.