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)

assigned to @wdconinc

changed the description

marked the checklist item podio needs cxxstd support (https://github.com/spack/spack/pull/30437) as completed

added 108 commits

• a3c5820c...b058fbf3 - 107 commits from branch master
• 6e7be14a - feat: root cxxstd=20

Compare with previous version

added 121 commits

• 6e7be14a...3d4de595 - 119 commits from branch master
• 94b5998d - chore: root-6.28.04
• 7152f96e - feat: root cxxstd=20

Compare with previous version

changed target branch from master to root-6.28.04

changed the description

marked the checklist item 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) as completed

added 11 commits

• 7152f96e...a856c2ab - 10 commits from branch root-6.28.04
• 630c4e37 - Merge branch 'root-6.28.04' into 'root-cxxstd-20'

Compare with previous version

added 1 commit

• 9a5491f8 - fix: acts cxxstd variant cherry-picks

Compare with previous version

added 1 commit

• b779b546 - fix: acts cxxstd=20

Compare with previous version

added 1 commit

• c59bbf7b - fix: podio cxxstd=20

Compare with previous version

changed the description

added 28 commits

• c59bbf7b...8b51e94c - 24 commits from branch root-6.28.04
• d7c72397 - feat: root cxxstd=20
• b81877f8 - fix: acts cxxstd variant cherry-picks
• a7f73414 - fix: acts cxxstd=20
• 02fb2a53 - fix: podio cxxstd=20

Compare with previous version

added 28 commits

• c59bbf7b...8b51e94c - 24 commits from branch root-6.28.04
• d7c72397 - feat: root cxxstd=20
• b81877f8 - fix: acts cxxstd variant cherry-picks
• a7f73414 - fix: acts cxxstd=20
• 02fb2a53 - fix: podio cxxstd=20

Compare with previous version

added 28 commits

• c59bbf7b...8b51e94c - 24 commits from branch root-6.28.04
• d7c72397 - feat: root cxxstd=20
• b81877f8 - fix: acts cxxstd variant cherry-picks
• a7f73414 - fix: acts cxxstd=20
• 02fb2a53 - fix: podio cxxstd=20

Compare with previous version

added 6 commits

• 02fb2a53...012f223b - 5 commits from branch root-6.28.04
• e853005c - feat: root,acts,podii,edm4hep,edm4eic cxxstd=20

Compare with previous version

marked the checklist item need support for acts@24: for cxxstd=20 (https://github.com/spack/spack/pull/37055) as completed

added 5 commits

• e853005c...3ba47155 - 4 commits from branch root-6.28.04
• 5c943a70 - feat: root,acts,podii,edm4hep,edm4eic cxxstd=20

Compare with previous version