Add backport workflow (#11691)

Mirrors dotnet/maui's setup. Maintainers can comment on a merged PR with:

    /backport to release/<branch>

to trigger the shared Arcade backport workflow, which cherry-picks the PR
to the target branch and opens a new PR (or comments with conflict
instructions if it cannot apply cleanly).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: jonathanpeppers

.github/workflows/backport.yml

@@ -0,0 +1,18 @@
+name: Backport PR to branch
+on:
+  issue_comment:
+    types: [created]
+  schedule:
+    # once a day at 13:00 UTC to cleanup old runs
+    - cron: '0 13 * * *'
+
+permissions:
+  contents: write
+  issues: write
+  pull-requests: write
+  actions: write
+
+jobs:
+  backport:
+    if: github.repository_owner == 'dotnet'
+    uses: dotnet/arcade/.github/workflows/backport-base.yml@main