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)

spack-environment/dev/spack.yaml

@@ -46,7 +46,7 @@ spack:
     - opencascade@7.7.1
     - osg-ca-certs@1.109.igtf.1.117
     - phonebook-cli@1.0.0
-    - podio@0.16.5
+    - podio@0.16.5 cxxstd=20
     - prmon@3.0.2 +plot
     - pythia8@8.309 +fastjet
     - python@3.10.10