Bump vega-embed from 6.6.0 to 6.20.8 in /packages/playground

Bumps [vega-embed](https://github.com/vega/vega-embed) from 6.6.0 to 6.20.8.
- [Release notes](https://github.com/vega/vega-embed/releases)
- [Changelog](https://github.com/vega/vega-embed/blob/v6.20.8/CHANGELOG.md)
- [Commits](https://github.com/vega/vega-embed/compare/v6.6.0...v6.20.8)

---
updated-dependencies:
- dependency-name: vega-embed
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

packages/playground/package.json

@@ -35,7 +35,7 @@
     "react-use": "^17.3.2",
     "react-vega": "^7.4.4",
     "vega": "*",
-    "vega-embed": "6.6.0",
+    "vega-embed": "6.20.8",
     "vega-lite": "*"
   },
   "devDependencies": {