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

@@ -3,7 +3,7 @@ spack:
   - ../concretizer.yaml
   - ../packages.yaml
   specs:
-    - acts@21.1.1 +dd4hep +examples +fatras +geant4 +identification +json +python +tgeo
+    - acts@21.1.1 cxxstd=20 +dd4hep +examples +fatras +geant4 +identification +json +python +tgeo
     - actsvg@0.4.26 +examples
     - acts-dd4hep@1.0.1
     - afterburner@0.1.2 +root +zlib
@@ -19,8 +19,8 @@ spack:
     - dawncut@1_54a
     - dd4hep@1.25.1 +ddg4 +ddcad -frames +hepmc3 +lcio +xercesc
     - dpmjet@19.3.5
-    - edm4eic@2.0.0 cxxstd=17
-    - edm4hep@=0.7 cxxstd=17
+    - edm4eic@2.0.0 cxxstd=20
+    - edm4hep@=0.7 cxxstd=20
     - eic-smear@1.1.10
     - eigen@3.4.0
     - emacs@28.2