[Http Client] Move retry_failed config inside retry_failed section

[hellomedia]

See issue #21434

This PR moves the http_client.retry_failed configuration inside that section, to align the doc with this:

$ bin/console debug:config framework

    http_client:
        default_options:
            retry_failed:
                enabled: false
                retry_strategy: null
                http_codes: {  }
                max_retries: 3
                delay: 1000
                multiplier: 2
                max_delay: 0
                jitter: 0.1

Eg:

framework.http_client.max_retries

becomes:

framework.http_client.retry_failed.max_retries

I am not sure if this is the proper way to go, as I did not see other sections in the document with this level of nesting, and I am not sure if the side menu handles it.