[LintDiff] Pin version of @microsoft.azure/openapi-validator-rulesets

LintDiff pins the version of `@microsoft.azure/openapi-validator`, but it does not pin the version of `@microsoft.azure/openapi-validator-rulesets`. `@microsoft.azure/openapi-validator` does not pin either. The end result is LintDiff is using `@microsoft.azure/openapi-validator-rulesets@latest`, which can break any time a new version is published.

Instead, LintDiff should somehow pin the version of `@microsoft.azure/openapi-validator-rulesets`, so it is only updated via a PR merged to LintDiff. LintDiff could pin the version directly, or `@microsoft.azure/openapi-validator` could pin instead of float.