build(deps-dev): bump graphql from 15.5.3 to 15.6.0 (#43)

Bumps [graphql](https://github.com/graphql/graphql-js) from 15.5.3 to 15.6.0.
- [Release notes](https://github.com/graphql/graphql-js/releases)
- [Commits](graphql/graphql-js@v15.5.3...v15.6.0)

---
updated-dependencies:
- dependency-name: graphql
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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Author: dependabot[bot]