Bump ajv from 6.11.0 to 6.12.6 in /products/scilla-server (#608)

Bumps [ajv](https://github.com/ajv-validator/ajv) from 6.11.0 to 6.12.6.
- [Release notes](https://github.com/ajv-validator/ajv/releases)
- [Commits](ajv-validator/ajv@v6.11.0...v6.12.6)

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

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>