This closes #10489. It adds `dependentKeyCompat` to the allocation getter so it works as expected as a dependent key for the `tracker` computed property, as described here: https://guides.emberjs.com/release/upgrading/current-edition/tracked-properties/#toc_backwards-compatibility
In addition to this computation being wasteful, it introduces a bug where the allocation on a stats tracker can update twice in one render, which isn't allowed in Glimmer (ironically, Glimmmer's lack of auto-memoization introduced the issue).