Add policy bot automation to assign milestones to pull requests based on target branch (#10684)

* Initial plan

* Add auto-assign milestone workflow with documentation

Co-authored-by: davidfowl <[email protected]>

* Replace Python implementation with GitHub CLI and bash scripting

- Eliminates Python dependency and setup step
- Uses built-in GitHub CLI (`gh`) for API calls
- Implements version parsing and comparison in bash
- Maintains same functionality and logic as original Python version
- Updates documentation to reflect new implementation approach

Co-authored-by: joperezr <[email protected]>

* Replace GitHub Actions workflow with policy bot automation

Co-authored-by: joperezr <[email protected]>

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: davidfowl <[email protected]>
Co-authored-by: joperezr <[email protected]>

Author: 3 people

.github/policies/milestoneAssignment.prClosed.yml

@@ -0,0 +1,31 @@
+id: milestoneAssignment.prClosed
+name: Auto Assign Milestone to Merged PRs
+description: Automatically assigns milestones to pull requests when they are merged based on target branch
+owner:
+resource: repository
+disabled: false
+where:
+configuration:
+  resourceManagementConfiguration:
+    eventResponderTasks:
+    - if:
+      - payloadType: Pull_Request
+      - isAction:
+          action: Closed
+      - targetsBranch:
+          branch: main
+      then:
+      - addMilestone:
+          milestone: 9.5
+      description: '[Milestone Assignments] Assign Milestone to PRs merged to the `main` branch'
+    - if:
+      - payloadType: Pull_Request
+      - isAction:
+          action: Closed
+      - targetsBranch:
+          branch: release/9.4
+      then:
+      - removeMilestone
+      - addMilestone:
+          milestone: 9.4.1
+      description: '[Milestone Assignments] Assign Milestone to PRs merged to release/9.4 branch'