Bump leveldown from 5.6.0 to 6.0.0 (#57)

Bumps [leveldown](https://github.com/Level/leveldown) from 5.6.0 to 6.0.0.
- [Release notes](https://github.com/Level/leveldown/releases)
- [Changelog](https://github.com/Level/leveldown/blob/master/CHANGELOG.md)
- [Commits](Level/leveldown@v5.6.0...v6.0.0)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

package.json

@@ -28,7 +28,7 @@
   "dependencies": {
     "encoding-down": "^7.0.0",
     "fastify-plugin": "^3.0.0",
-    "leveldown": "^5.6.0",
+    "leveldown": "^6.0.0",
     "levelup": "^4.4.0"
   },
   "devDependencies": {