Change pr.yml -> sb-pr.yml

This allows us to repoint the SB pipeline at this YAML file, meaning that we can manage triggers for UB, which uses ci.yml, via YAML files rather than the UI. Updated comments to reflect where this file will be used.

Author: ViktorHofer

src/SourceBuild/content/eng/pipelines/pr.yml renamed to src/SourceBuild/content/eng/pipelines/sb-pr.yml

@@ -1,18 +1,11 @@
-# This yml is used by these PR pipelines and triggers:
-# NOTE: the triggers are defined in the Azure DevOps UI as they are too complex
-#
-# - dotnet-source-build (public)
-#   https://dev.azure.com/dnceng-public/public/_build?definitionId=240
-#   - PR: ultralite build
-#   - CI: release/* only, every batched commit, full build
-#   - Schedule: main only, full build
-#
-# - dotnet-unified-build (public)
-#   https://dev.azure.com/dnceng-public/public/_build?definitionId=278
-#   - PR: lite build
-#   - CI: release/* only, every batched commit, full build
-#   - Schedule: main only, full build
-#
+# This is the non-1ES PR pipeline for dotnet/dotnet
+# https://dev.azure.com/dnceng-public/public/_build?definitionId=240
+trigger: none
+pr:
+  branches:
+    include:
+    - release/9.0.2xx
+    - internal/release/9.0.2xx
 
 variables:
 # enable source-only build for pipelines that contain the -source-build string