* api: revert to defaulting to http/1

PR #10778 incidentally changed the api http client to connect with
HTTP/2 first. However, the websocket libraries used in `alloc exec`
features don't handle http/2 well, and don't downgrade to http/1
gracefully.

Given that the switch is incidental, and not requested by users.
Furthermore, api consumers can opt-in to forcing http/2 by setting
custom http clients.

Fixes #10922