Fix broken README supporter links

[lonelyhty]

Summary

• Replace the broken Open Collective contributors image with a working contributors image.
• Remove the broken backer 9 avatar link.
• Remove the sponsor 9 link that redirects to a deleted GitHub account.

Context

Closes #2733.

Validation

• Verified https://opencollective.com/solid/backer/9/avatar.svg returns 404.
• Verified https://opencollective.com/solid/sponsor/9/website redirects to a 404 GitHub profile.
• Verified the replacement contributors image returns HTTP 200 with image/svg+xml.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 3d0cccd

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