Cannot set ajv instace to 2019 and 2020

[nicobuzeta]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Fastify version

4.26.1

Plugin version

2.5.1

Node.js version

21.7.1

Operating system

Linux

Operating system version (i.e. 20.04, 11.3, 10)

Docker node:latest

Description

Current plugin does not allow ajv instances created with non-draft 7 schemas to be used like:
ajv/dist/2019
ajv/dist/2020

Steps to Reproduce

import ajvNew from 'ajv/dist/2020.js';

export default fp(async (fastify) => {
  const ajv = new ajvNew.default();
  await fastify.register(response_validation, {
    ajv
  });
}


### Expected Behavior

Expect it to use my ajv instance internally. Right now the check for setting the ajv instance is:

if (opts.ajv && opts.ajv instanceof Ajv) {

However, this fails for this case since the ajv I am passing in is not of type Ajv.

[nicobuzeta]

I've proposed a possible fix in #108