chore(deps): update dependency ajv to v6.9.0

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change	References
ajv	devDependencies	minor	6.8.1 -> 6.9.0	source

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Release Notes

epoberezkin/ajv

### [`v6.9.0`](https://togithub.com/epoberezkin/ajv/releases/v6.9.0) [Compare Source](https://togithub.com/epoberezkin/ajv/compare/v6.8.1...v6.9.0) OpenAPI keyword `nullable` can be any boolean (and not only `true`). Custom keyword definition changes: - `dependencies` option in to require the presence of keywords in the same schema. - more strict validation of the definition using JSON Schema.

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