[CP-beta]Run the mac_unopt sub-builds as standalone builders - #196058

`Mac mac_unopt` is an orchestrator. It schedules 8 Mac sub-builds, and
holds a Mac bot for the 43 minutes they take.

Instead, this PR moves each sub-build to a top-level engine_v2/builder
target, which is how `windows_unopt` has run since
flutter-team-archive/engine#41785. The build definitions are
copied unchanged into ci/builders/standalone/.

The new targets start in bringup and a follow-up PR will delete Mac
mac_unopt and mac_unopt.json.

With this change each builder will have its own history on the
dashboard, so a failure shows which build broke rather than "mac_unopt".
When one flakes, only that one reruns, ditto suppressions. When there's
a flake it will be much easier to spelunk into the logs to figure out
what's happening (see flutter#185908).

The orchestrator's bot goes away (around ~300 Mac bot-hours / wk).
Unfortunately wall time doesn't change because the sub-builds were
already running in parallel (though it should be easier to make changes
in the future that improve wall time). This will also add 8 new squares
to the dashboard. Will also need release branch cherry-picks to get the
infra configs to match.

This shouldn't impact building/uploading engine binaries or anything in
the merge queue (afaik).

Additional history:

- Orchestrator created:
flutter-team-archive/engine#40860
- Orchestrator moved to Linux to save a Mac builder:
flutter-team-archive/engine#41184 and
flutter-team-archive/engine#41226
- Moved back to Mac: flutter-team-archive/engine#50863 (for
flutter#142010) because the name was
confusing

Part of  flutter#155041
Part of flutter#186462


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The standalone builders added in flutter#195868 are passing in post-submit.
Remove bringup from them and delete the old mac_unopt orchestrator
builder and its config. Update docs that referenced mac_unopt.json.

(cherry picked from commit 84d0eb2)