docs(changelog): reformat 2nd-gen CHANGELOGs to match strategy

[Rajdeepc]

Summary

• Reformatted @adobe/spectrum-wc and @spectrum-web-components/core CHANGELOGs to match the agreed changelog strategy
• Removed author attributions, deduplicated core entries, added missing component docs and migration guide links

Test plan

• Verify CHANGELOG entries render correctly on GitHub

[changeset-bot]

⚠️ No Changeset found

Latest commit: dc2b27f

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-actions]

📚 Branch Preview Links

• Documentation Site (first-gen)
• Storybook (first-gen)
• Storybook (second-gen)

🔍 First Generation Visual Regression Test Results

When a visual regression test fails (or has previously failed while working on this branch), its results can be found in the following URLs:

• Spectrum | Light | Medium | LTR
• Spectrum | Dark | Large | RTL
• Express | Light | Medium | LTR
• Express | Dark | Large | RTL
• Spectrum-two | Light | Medium | LTR
• Spectrum-two | Dark | Large | RTL
• High Contrast Mode | Medium | LTR

Deployed to Azure Blob Storage: pr-6336

If the changes are expected, update the current_golden_images_cache hash in the circleci config to accept the new images. Instructions are included in that file.
If the changes are unexpected, you can investigate the cause of the differences and update the code accordingly.