chore(deps): bump ajv and react-jsonschema-form in /superset-frontend

Bumps [ajv](https://github.com/ajv-validator/ajv) to 6.12.6 and updates ancestor dependency [react-jsonschema-form](https://github.com/mozilla-services/react-jsonschema-form). These dependencies need to be updated together.


Updates `ajv` from 5.5.2 to 6.12.6
- [Release notes](https://github.com/ajv-validator/ajv/releases)
- [Commits](ajv-validator/ajv@v5.5.2...v6.12.6)

Updates `react-jsonschema-form` from 1.2.0 to 1.8.1
- [Release notes](https://github.com/mozilla-services/react-jsonschema-form/releases)
- [Changelog](https://github.com/rjsf-team/react-jsonschema-form/blob/main/CHANGELOG.md)
- [Commits](rjsf-team/react-jsonschema-form@v1.2.0...v1.8.1)

---
updated-dependencies:
- dependency-name: ajv
  dependency-type: indirect
- dependency-name: react-jsonschema-form
  dependency-type: direct:production
...

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