fix: rollback ajv version to 6.12.6

src/validate.ts

@@ -39,7 +39,7 @@ export class Validator {
   /**
    * JSON Schema Validator
    */
-  private readonly ajv = new Ajv({verbose: true});
+  private readonly ajv = Ajv({verbose: true, jsonPointers: true, extendRefs: true});
   /**
    * Map of schema names to schemas
    */
@@ -134,7 +134,7 @@ function fromAjvResult(
   result: boolean | PromiseLike<unknown>,
   schema: JSONSchema7,
   instance: unknown,
-  ajvInstance: Ajv,
+  ajvInstance: Ajv.Ajv,
 ): ValidationResult {
   // tslint:disable-next-line
   // @ts-ignore function can return void, which at runtime will be undefined. TS doesn't allow to assign void to undefined