bot: Bump graphiql from 3.7.2 to 3.8.1 in /playground

Bumps [graphiql](https://github.com/graphql/graphiql/tree/HEAD/packages/graphiql) from 3.7.2 to 3.8.1.
- [Release notes](https://github.com/graphql/graphiql/releases)
- [Changelog](https://github.com/graphql/graphiql/blob/main/packages/graphiql/CHANGELOG.md)
- [Commits](https://github.com/graphql/graphiql/commits/[email protected]/packages/graphiql)

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

Signed-off-by: dependabot[bot] <[email protected]>