Editor: Use Stack for post summary

[ntsekouras]

What?

This PR just uses Stack for ClassicPostSummary and the DataFormPostSummary components.

Testing Instructions

1. Summary component should be identical to trunk
2. Same goes for the summary component under Editor Inspector: Use DataForm experiment

[github-actions]

Size Change: +9 B (0%)

Total Size: 7.5 MB

📦 View Changed

Filename	Size	Change
build/scripts/editor/index.min.js	473 kB	+9 B (0%)

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[github-actions]

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[github-actions]

Flaky tests detected in 0de6abc.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/27949893856
📝 Reported issues:

• [Flaky Test] should update the HTML only for the latest navigation #78835 in /test/e2e/specs/interactivity/router-navigate.spec.ts
• [Flaky Test] three users concurrently edit a large post with diverse blocks #78754 in /test/e2e/specs/editor/collaboration/collaboration-stress.spec.ts