Remove group as allowed child of tree

[mcking65]

Closes #2014

Removes group as an allowed accessibility child of tree to fix inconsistency with definition of treeitem. Direct children of tree can only be parent nodes or end nodes. Leaf nodes must be a child of a group that is a child of a parent node treitem. Even in a virtualized tree, if the elements that represent the ancestry of rendered nodes were not present in the DOM, the tree would be inoperable.

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[giacomo-petri]

Since I approved the PR without adding any notes (and changes seem great based on the rationale of #2315), I'm assuming I mistakenly applied the agenda label to the wrong GitHub PR. Now, it would be interesting to figure out where I missed applying the agenda label... 🥲