Bump ajv from 5.5.2 to 6.12.6

Bumps [ajv](https://github.com/ajv-validator/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)

---
updated-dependencies:
- dependency-name: ajv
  dependency-type: indirect
...

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Author: dependabot[bot]