Turn on strict validation

What does this MR do?

Turns on strict-mode for ajv validation. The following errors needed to be corrected before the schema would pass:

• self keyword is not a reserved json-schema keyword
• object type needs to exist when one is missing and require is true
• ajv does not know about formats like uri-reference out of the box
• union types need to be enabled if using keywords like oneOf
• minLength requires a type of string

You can evaluate each fix separately by going to the appropriate commit.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/security-products/security-report-schemas/-/issues/9

Checklist

• Ensure changes can be built upon without requiring a breaking change, see Building for Extensibility.
• Review and add/update tests for this feature/bug.
• Add an entry to the CHANGELOG if required, with the appropriate version. See Classifying Changes.
• Assign the MR to the appropriate person/people for review.