🔥 Exterminate legacy `backport` GHA workflow

Previously, this workflow was being used to cherry-pick PRs made
against `main` into older stable branches. Now that #12475 integrated
the Patchback GitHub App, it's no longer needed as it's making
duplicate pull requests which don't even trigger CI runs automatically
due to #10354. So this patch removes said workflow to address the
problem.

.github/workflows/backport.yml

@@ -1,51 +0,0 @@
-name: backport
-
-on:
-  # Note that `pull_request_target` has security implications:
-  # https://securitylab.github.com/research/github-actions-preventing-pwn-requests/
-  # In particular:
-  # - Only allow triggers that can be used only be trusted users
-  # - Don't execute any code from the target branch
-  # - Don't use cache
-  pull_request_target:
-    types: [labeled]
-
-# Set permissions at the job level.
-permissions: {}
-
-jobs:
-  backport:
-    if: startsWith(github.event.label.name, 'backport ') && github.event.pull_request.merged
-    runs-on: ubuntu-latest
-    permissions:
-      contents: write
-      pull-requests: write
-
-    steps:
-    - uses: actions/checkout@v4
-      with:
-        fetch-depth: 0
-        persist-credentials: true
-
-    - name: Create backport PR
-      run: |
-        set -eux
-
-        git config --global user.name "pytest bot"
-        git config --global user.email "pytestbot@gmail.com"
-
-        label='${{ github.event.label.name }}'
-        target_branch="${label#backport }"
-        backport_branch=backport-${{ github.event.number }}-to-"${target_branch}"
-        subject="[$target_branch] $(gh pr view --json title -q .title ${{ github.event.number }})"
-
-        git checkout origin/"${target_branch}" -b "${backport_branch}"
-        git cherry-pick -x --mainline 1 ${{ github.event.pull_request.merge_commit_sha }}
-        git commit --amend --message "$subject"
-        git push --set-upstream origin --force-with-lease "${backport_branch}"
-        gh pr create \
-          --base "${target_branch}" \
-          --title "${subject}" \
-          --body "Backport of PR #${{ github.event.number }} to $target_branch branch. PR created by backport workflow."
-      env:
-        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}