build: Bump g++ minimum supported version to 13 #36042

At some point, it may be fine to drop support for older vanilla systems not shipping with at least GCC 13.

(For this bump, only C++20 std::format will become available, but this bump should also pave the way for C++23.)

This patch will only be released in version 33.x, or later, next year (2027).

For reference:

• https://packages.ubuntu.com/noble/g++ (g++-13)
• https://packages.debian.org/trixie/g++ (g++-14)
• CentOS-like 8/9/10 ship recent GCC via appstream (dnf install -y gcc-toolset-15)
• Free/Open/Net*BSD ship a recent GCC
• OpenSuse Leap and Tumbleweed ship with g++ 15 https://software.opensuse.org/package/gcc15-c++

On operating systems where the minimum GCC version is not shipped by default, the user would have to use Clang, or install GCC in a different way. For example:

• Debian Bookworm could use https://packages.debian.org/bookworm/clang-22
• Ubuntu 22.04 would have to use a general alternative:
• PPA, https://apt.llvm.org/ (for Clang), brew, nix, guix, or compile GCC from source, ...

Obviously, downloading pre-compiled releases or compiling previous release branches is unaffected by this change.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/36042.

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🚧 At least one of the CI tasks failed. _{Task NetBSD Cross: https://github.com/bitcoin/bitcoin/actions/runs/32388090156/job/96487376676} _{LLM reason (✨ experimental): CI failed because CMake’s try-compile for text formatting errored with fatal error: 'format' file not found (missing C++ <format> header).}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

</details>

Hmm, looks like netbsd-cross is a CI blocker for this? Not sure how to proceed here, because netbsd ci coverage may be useful, but no real user is doing netbsd-cross (I expect all users are running on native netbsd). But native netbsd isn't an option for CI ... :man_shrugging: