Document XL Cirrus Runners (#613)

* Document XL Cirrus Runners

Also tried to put everything about Cirrus Runners in one place rather than having the information spreaded between https://tart.run and https://github.com/apps/cirrus-runners.

Plus updated docs to use Sonoma.

* Apply suggestions from code review

Co-authored-by: Nikolay Edigaryev <edigaryev@gmail.com>

---------

Co-authored-by: Nikolay Edigaryev <edigaryev@gmail.com>

docs/integrations/cirrus-cli.md

@@ -13,7 +13,7 @@ task:
   name: hello
   macos_instance:
     # can be a remote or a local virtual machine
-    image: ghcr.io/cirruslabs/macos-ventura-base:latest
+    image: ghcr.io/cirruslabs/macos-sonoma-base:latest
   hello_script:
     - echo "Hello from within a Tart VM!"
     - echo "Here is my CPU info:"
@@ -45,7 +45,7 @@ exposes it via [`artifacts` instruction](https://cirrus-ci.org/guide/writing-tas
 task:
   name: Build
   macos_instance:
-    image: ghcr.io/cirruslabs/macos-ventura-xcode:latest
+    image: ghcr.io/cirruslabs/macos-sonoma-xcode:latest
   build_script: swift build --product tart
   binary_artifacts:
     path: .build/debug/tart