Bump ajv and grunt-jsonlint

Bumps [ajv](https://github.com/ajv-validator/ajv) to 6.12.6 and updates ancestor dependency [grunt-jsonlint](https://github.com/brandonramirez/grunt-jsonlint). These dependencies need to be updated together.


Updates `ajv` from 6.10.2 to 6.12.6
- [Release notes](https://github.com/ajv-validator/ajv/releases)
- [Commits](ajv-validator/ajv@v6.10.2...v6.12.6)

Updates `grunt-jsonlint` from 2.1.3 to 3.0.0
- [Commits](brandonramirez/grunt-jsonlint@v2.1.3...v3.0.0)

---
updated-dependencies:
- dependency-name: ajv
  dependency-type: indirect
- dependency-name: grunt-jsonlint
  dependency-type: direct:development
...

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