Replace `OkHttp3ClientHttpRequestFactory` with `JdkClientHttpRequestFactory`

[BoykoAlex]

The org.springframework.ide.vscode.boot.app.RestTemplateFactory uses deprecated org.springframework.http.client.OkHttp3ClientHttpRequestFactory. Replace is with org.springframework.http.client.JdkClientHttpRequestFactory.

The trick would be testing the proxy settings with this as it would probably require some kind of setup.

[BoykoAlex]

Fixed with 57510f8