Move to C++20 and remove vector_utils_legacy

[jmcarcell]

for the next release of the Key4hep stack we may want to consider moving to C++20 so that C++20 features can be used without automatically introducing incompatibilities, and they are easier to adopt if we are already using C++20. This would help make the release usable for a longer time. The Key4hep stack has been built with C++20 for many months by now. There are the LCG stacks on C++17 as far as I know and it's possible other people are using, for example, an older version of ROOT that can't be built with C++20 in which case this may have to wait.

BEGINRELEASENOTES

• Move to C++20 and remove vector_utils_legacy

ENDRELEASENOTES

[tmadlener]

CI probably needs some adjustments, because some of the workflows build explicitly with c++17, I think.

[tmadlener]

Can we keep one LCG release and an ubuntu build and just build them with c++20?

[andresailer]

gcc13 is build with c++20 (so is clang16). Nothing on ubuntu yet.

[tmadlener]

I think gcc11 on ubuntu22.04 should have enough c++20 capabilities for EDM4hep.

[tmadlener]

Looks like LCG 106 on Ubuntu 22.04 does not have a c++20 capable ROOT yet? https://github.com/key4hep/EDM4hep/actions/runs/10486852425/job/29045992210?pr=343#step:4:751

edit: it doesn't:

$ source /cvmfs/sft.cern.ch/lcg/views/LCG_106/x86_64-ubuntu2204-gcc11-opt/setup.sh
$ root-config --features
cxx17 asimage builtin_afterimage builtin_clang builtin_cling builtin_ftgl builtin_glew builtin_llvm builtin_lz4 builtin_openui5 builtin_pcre builtin_unuran builtin_xxhash builtin_zstd clad dataframe davix fftw3 fitsio fortran gdml http imt mathmore mysql opengl pyroot roofit webgui root7 rpath runtime_cxxmodules shared sqlite ssl tmva tmva-cpu tmva-sofie tmva-pymva spectrum unfold unuran vc vdt x11 xml xrootd

[andresailer]

Only LCG stacks with gcc13 and clang16 or newer have c++20 enabled.

[jmcarcell]

Ubuntu removed, can be added back if it has C++20, anyway the Key4hep builds have Ubuntu.