main.star: bump xcode versions, again

Re-applying CL 572595, after the revert in CL 573077.

Fixes golang/go#66394

Change-Id: I1f7191c023dfefbdcc37b1340269ae2e3d876c8f
Reviewed-on: https://go-review.googlesource.com/c/build/+/582815
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Auto-Submit: Roland Shoemaker <[email protected]>

Author: rolandshoemaker