build(deps): bump ajv from 6.10.2 to 6.12.6 in /generator/javascript

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

)

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

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