docs: update upstream-integration flow for branch protection

Merge integrate-upstream-changes PRs via GitHub 'Create a merge commit'
instead of a local fast-forward + direct push. Branch protection on
localstack now blocks direct/force pushes, and squash-only would collapse
upstream commits; a merge commit preserves upstream history and goes
through a PR.

Author: carole-lavillonniere

README-LOCALSTACK.md

@@ -34,8 +34,7 @@ Follow these steps to integrate upstream changes from the official AWS [lambda-r
 4. Merge the upstream changes from develop into the new branch `git merge develop` and resolve any potential conflicts.
 5. If needed, add a single commit with minimal changes to adjust the localstack customizations to the new changes.
 6. Create a PR on Github against `localstack/lambda-runtime-init localstack` (️not against AWS as by default ⚠️).
-7. **MERGING:** Manually merge the approved PR using `git checkout localstack && git merge --ff integrate-upstream-changes` and add the PR number as a suffix to the commit message. Example: `(#24)`. Do not squash any upstream commits!
-8. Manually push `git push origin localstack` and close the PR on GitHub
+7. **MERGING:** Merge the approved PR on GitHub using **"Create a merge commit"** (⚠️ **not** "Squash and merge"). Squashing collapses the upstream commit history into a single commit, which breaks conflict resolution on future `git merge develop` runs. The merge commit preserves the individual upstream commits and satisfies branch protection on `localstack` (which blocks direct/force pushes), so no manual `git push` is needed.
 
 Example PR that integrates upstream changes: https://github.com/localstack/lambda-runtime-init/pull/24