chore: Fix invalid PR number in eth-block-tracker changelog

[Mrtenz]

Explanation

The eth-block-tracker changelog accidentally contained an invalid PR number, which has been fixed in this PR.

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed, highlighting breaking changes as necessary
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes

---

Note

Corrects PR numbers in packages/eth-block-tracker/CHANGELOG.md from #7930 to #6865 for the Node.js minimum version change and monorepo migration notes.

• Changelog (packages/eth-block-tracker/CHANGELOG.md):
  • Correct PR references from #7930 to #6865 for:
    • The breaking change updating minimum Node.js version
    • The migration to the MetaMask/core monorepo

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