Revert "ci: use BACKPORT_MERGE_COMMIT option (#16730)" (#17116)

This reverts commit 1721e68.

The change was expected to solve the sporadic problems we were having
with Backport Assistant, but it end up creating even more failures.

.github/workflows/backport.yml

@@ -19,15 +19,17 @@ jobs:
         env:
           BACKPORT_LABEL_REGEXP: "backport/(?P<target>website)"
           BACKPORT_TARGET_TEMPLATE: "stable-{{.target}}"
-          BACKPORT_MERGE_COMMIT: true
+          # Enabling this option increased the number of backport failures.
+          BACKPORT_MERGE_COMMIT: false
           GITHUB_TOKEN: ${{ secrets.ELEVATED_GITHUB_TOKEN }}
       - name: Backport changes to targeted release branch
         run: |
           backport-assistant backport -merge-method=squash -automerge
         env:
           BACKPORT_LABEL_REGEXP: "backport/(?P<target>\\d+\\.\\d+\\.[+\\w]+)"
           BACKPORT_TARGET_TEMPLATE: "release/{{.target}}"
-          BACKPORT_MERGE_COMMIT: true
+          # Enabling this option increased the number of backport failures.
+          BACKPORT_MERGE_COMMIT: false
           GITHUB_TOKEN: ${{ secrets.ELEVATED_GITHUB_TOKEN }}
   handle-failure:
     needs: