chore: remove `backport-docs` (#14978)

- this is in support of removal of `stable-website`

.github/workflows/backport-docs.yml

@@ -1,46 +0,0 @@
----
-name: Backport Assistant Runner For Docs (OSS only)
-
-on:
-  pull_request_target:
-    types:
-      - closed
-      - labeled
-
-jobs:
-  backport-for-docs:
-    if: github.event.pull_request.merged
-    runs-on: ubuntu-latest
-    container: hashicorpdev/backport-assistant:0.2.3
-    steps:
-      - name: Backport changes to stable-website
-        run: |
-          backport-assistant backport -automerge -merge-method=squash
-        env:
-          BACKPORT_LABEL_REGEXP: "backport/(?P<target>website)"
-          BACKPORT_TARGET_TEMPLATE: "stable-{{.target}}"
-          GITHUB_TOKEN: ${{ secrets.ELEVATED_GITHUB_TOKEN }}
-      - name: Backport changes labeled website to latest release branch
-        if: always()
-        run: |
-          resp=$(curl -f -s "https://api.github.com/repos/$GITHUB_REPOSITORY/releases?per_page=100")
-          ret="$?"
-          if [[ "$ret" -ne 0 ]]; then
-              echo "The GitHub API returned $ret"
-              exit $ret
-          fi
-
-          # find the latest non-rc release
-          latest_version=$(echo "$resp" | tr '\r\n' ' ' | jq -r '.[] | select(.name|test("^v\\d+\\.\\d+\\.\\d+$")) | .name' | sort -rV | head -n1)
-          echo "Latest non-rc version: $latest_version"
-
-          # strip leading "v" &
-          target="${latest_version#v}"
-          # replace patch version with "x"
-          target="${target%.*}.x"
-
-          export BACKPORT_TARGET_TEMPLATE="release/$target"
-          backport-assistant backport
-        env:
-          BACKPORT_LABEL_REGEXP: "backport/(?P<target>website)"
-          GITHUB_TOKEN: ${{ secrets.ELEVATED_GITHUB_TOKEN }}