ci: remove automerge from upgrade workflow

[marandaneto]

💡 Motivation and Context

The organization blocks auto-merge. The SDK upgrade workflow currently tries to enable it on the pull requests it creates in PostHog/posthog, so that step cannot succeed.

This removes the auto-merge command. The workflow will continue to create the pull request and assign the SDK reviewer teams.

💚 How did you test it?

• Ran actionlint .github/workflows/posthog-upgrade.yml.
• Ran git diff --check.

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file

[greptile-apps]

_{Reviews (1): Last reviewed commit: "ci: remove automerge from upgrade workfl..." | Re-trigger Greptile}