Move away from deprecated 10.15 runners for ios tests (#81903)
GH is deprecating these and failing a bunch of workflows today: https://hud.pytorch.org/pytorch/pytorch/commit/8d0cbce0696e76c9e4c929c24c0331df25125b40
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81903
Approved by: https://github.com/seemethere
