This reverts commit 7d3b1363.

* Impl

* Macos

This is to keep all the mac machines on the same version. Mac machines
are being updated to Mac 12.0.1 to support iOS 15 phones as they are
getting forced updated.

Bug: https://github.com/flutter/flutter/issues/93667

* Update the mac_ios group to include newer dimension values for macOS, xocde and iOS.

This allows us to add a couple updated testbeds to the prod pool, and run jobs on them as well.
Once they are validated, we will move to update all testsbeds in prod and remove the older values
from the .ci.yaml file.

Bug:94767

* We are going to swap out the fleet in one shot since it doesn't seem to be possible to provide multi-values for these dimensions.

This reverts commit c567f843.