Docs: Update to reflect new pipeline (#8429)

A new windows self-hosted pipeline was enabled. Update the
`CONTRIBUTING.md` and the `Utilities/README.md` to indicate how to
trigger the new pipeline, and how to reproduce the self-hosted pipeline
at-desk.

Author: bkhouri

Diff for: CONTRIBUTING.md

@@ -350,6 +350,11 @@ To run just the self-hosted test suite (faster turnaround times so it can be use
 @swift-ci please test self hosted
 ```
 
+To run the windows self-hosted suite, use:
+
+```
+@swift-ci please test self hosted windows
+```
 
 To run the swift toolchain test suite including SwiftPM use:
 

Diff for: Utilities/README.md

@@ -14,9 +14,9 @@ Here are some steps the worked running on Windows.
     cd C:\source
     git clone https://github.com/swiftlang/swift-package-manager .
     # Assign the PR ID to a variable
-    $PR_ID = "8210"
-    git fetch origin pull/$PR_ID/merge:ci_merge_$PR_ID
-    git checkout ci_merge_$PR_ID
+    $PR_ID = "8288"
+    git fetch origin pull/$PR_ID/merge
+    git checkout FETCH_HEAD
     ```
 5. Run the CI pipeline script
     ```