Move to building everything with C++ 20

[timwoj]

As part of https://github.com/zeek/zeek/issues/4465, we realized that building with Node v23 also requires building with C++20. This is a good opportunity to upgrade to this version of C++ across the board. This PR is the Zeek side of things, which surprisingly required very few changes. This is only the infrastructure changes and anything that was required to make everything compile and btests pass. There are no changes to use C++20 features (such as removing zeek::Span for std::span). Those will come later once this has merged.

PRs for all of the submodules will be opened later as this PR stabilizes.