add github action to assign milestone automatically (nvaccess#16779)

NV Access has a server side script to automatically assign milestones to closed issues and PRs. This script has been malfunctioning for some time meaning we are not tracking which release issues are closed in.

Description of development approach
Used copilot to generate a github action to assign milestones.

.github/workflows/assign-milestone-on-close.yml

@@ -0,0 +1,41 @@
+name: Assign Milestone on Close
+
+env:
+  MILESTONE_ID: ${{ vars.MILESTONE_ID }}
+
+on:
+  issues:
+    types: [closed]
+  pull_request:
+    types: [closed]
+
+jobs:
+  assign-milestone:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check if milestone is set
+        id: check-milestone
+        uses: actions/github-script@v7
+        with:
+          github-token: ${{secrets.GITHUB_TOKEN}}
+          script: |
+            const issueOrPr = context.payload.issue || context.payload.pull_request;
+            if (!issueOrPr.milestone) {
+              core.setOutput('milestoneNotSet', 'true');
+            } else {
+              core.setOutput('milestoneNotSet', 'false');
+            }
+
+      - name: Assign default milestone
+        if: steps.check-milestone.outputs.milestoneNotSet == 'true'
+        uses: actions/github-script@v7
+        with:
+          github-token: ${{secrets.GITHUB_TOKEN}}
+          script: |
+            const issueOrPrNumber = (context.payload.issue || context.payload.pull_request).number;
+            const repository = context.repo;
+            await github.rest.issues.update({
+              ...repository,
+              issue_number: issueOrPrNumber,
+              milestone: process.env.MILESTONE_ID
+            });