Extend the luci bot swarming dimension to cover iOS-15, instead of needing to specify `iOS-15.1` https://github.com/flutter/flutter/blob/6a88c221d5a89ce5104a80df7b4a7c5be259f550/.ci.yaml#L101 See also https://github.com/flutter/flutter/pull/104727
