Bump ajv and @vue/cli-plugin-eslint

Bumps [ajv](https://github.com/ajv-validator/ajv) to 6.12.6 and updates ancestor dependency [@vue/cli-plugin-eslint](https://github.com/vuejs/vue-cli/tree/HEAD/packages/@vue/cli-plugin-eslint). 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 `@vue/cli-plugin-eslint` from 3.12.1 to 5.0.8
- [Release notes](https://github.com/vuejs/vue-cli/releases)
- [Changelog](https://github.com/vuejs/vue-cli/blob/dev/CHANGELOG.md)
- [Commits](https://github.com/vuejs/vue-cli/commits/v5.0.8/packages/@vue/cli-plugin-eslint)

---
updated-dependencies:
- dependency-name: ajv
  dependency-type: indirect
- dependency-name: "@vue/cli-plugin-eslint"
  dependency-type: direct:development
...

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Author: dependabot[bot]