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

@@ -75,7 +75,7 @@ spack:
     - py-uproot@5.0.5
     - py-yapf@0.30.0
     - qt@5.15.9 +opengl
-    - root@6.28.04 cxxstd=17 
+    - root@6.28.04 cxxstd=20 
           +fftw +fortran +gdml +http -ipo +mlp +pythia8 
           +root7 +tmva +vc -webgui +xrootd +ssl 
     - spdlog@1.11.0 +fmt_external