feat: ROOT C++20 support, `root cxxstd=20`

This adds ROOT C++20 support. It requires some changes to dependencies too:

• podio needs cxxstd support (https://github.com/spack/spack/pull/30437)
• acts hard-depends on root cxxstd=17 for no real reason other than inability to depend on multiple variants in spack (https://github.com/spack/spack/pull/38682)
• need support for acts@24: for cxxstd=20 (https://github.com/spack/spack/pull/37055)