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.sh

@@ -11,9 +11,13 @@ read -r -d '' SPACK_CHERRYPICKS <<- \
 9ee2d79de172de14477a78e5d407548a63eea33a
 776ab132760d63eab0703b7c0ebebc72a8443f5b
 188168c476eabe99764634db8d78eb3f9ea2a927
+ce0b9ea8cf184c9048cac1ae88f2d69f0e4520c7
+ea1439dfa11a3996c9927ed792dc9fe4b7efc1b8
 ---
 ## Ref: https://github.com/spack/spack/commit/[hash]
 ## [hash]: [description]
 ## 9ee2d79de172de14477a78e5d407548a63eea33a: libxpm package: fix RHEL8 build with libintl
 ## 776ab132760d63eab0703b7c0ebebc72a8443f5b: [xrootd] New variants, new version, improve build config
 ## 188168c476eabe99764634db8d78eb3f9ea2a927: podio: Add 0.16.5 tag
+## ce0b9ea8cf184c9048cac1ae88f2d69f0e4520c7: acts: ensure Python_EXECUTABLE uses ^python when +python
+## ea1439dfa11a3996c9927ed792dc9fe4b7efc1b8: acts: new variant cxxstd