module depends on deprecated "request" which has issues with certs

My BTCPayServer is using a letsencrypt cert which is affected by [this issue](https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021/). As explained in the link, the browser has no issue with expired certs but my api call fails.

```
$ node
> const key = "redacted"
> const btcpay = require('btcpay')
> const keypair = btcpay.crypto.load_keypair(new Buffer.from(key, 'hex'))
> const client = new btcpay.BTCPayClient("https://pos.btcpay.nz", keypair, {merchant: 'redacted'})
> client.get_invoices({status: "complete"}).then( invoices => {})
Promise { }
> (node:2032689) UnhandledPromiseRejectionWarning: RequestError: Error: certificate has expired
at new RequestError (/path/to/project/node_modules/request-promise-core/lib/errors.js:14:15)
at Request.plumbing.callback (/path/to/project/node_modules/request-promise-core/lib/plumbing.js:87:29)
at Request.RP$callback [as _callback] (/path/to/project/node_modules/request-promise-core/lib/plumbing.js:46:31)
at self.callback (/path/to/project/node_modules/request/request.js:185:22)
at Request.emit (events.js:193:13)
at Request.EventEmitter.emit (domain.js:481:20)
at Request.onRequestError (/path/to/project/node_modules/request/request.js:877:8)
at ClientRequest.emit (events.js:193:13)
at ClientRequest.EventEmitter.emit (domain.js:481:20)
at TLSSocket.socketErrorListener (_http_client.js:397:9)
at TLSSocket.emit (events.js:193:13)
at TLSSocket.EventEmitter.emit (domain.js:481:20)
at emitErrorNT (internal/streams/destroy.js:91:8)
at emitErrorAndCloseNT (internal/streams/destroy.js:59:3)
at processTicksAndRejections (internal/process/task_queues.js:81:17)
(node:2032689) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:2032689) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
```

As the crash is caused in `btcpay.BTCPayClient`, I suspect this should be fixed in this project?