Broken BACKERS.md link in sponsorship policy

Describe the bug

SPONSORS.md links to BACKERS.md for the Backer tier, but BACKERS.md is absent from the default branch.

To reproduce

1. Open SPONSORS.md at a56b4b8ad43a3767c771953d217036813f697bc7.
2. Follow the BACKERS.md link in the Backer tier.

Expected behavior

The link should resolve to the intended maintained backer registry, or the policy should not link to a file that is unavailable.

Actual behavior

The link resolves to GitHub's 404 response. The same document also refers to BACKERS.md later in the file.

Additional context

I checked the current default branch, open and closed issues/PRs, and discussions for an existing fix before filing. I am reporting the broken-link evidence only; the content and publication policy for a backer registry need maintainer direction.