[whatwg/fetch] Consider restricting 103 Early Hints to HTTP/2 or later (Issue #1698)

As mentioned in various places (see references below), informational responses like 103 Early Hints were previously not common, and may cause issues for clients that do not expect these. Should we restrict 103 Early Hints in the browser context to HTTP/2 or later as most implementations have done so far?

- 103 Early Hints [RFC 8297 Security Considerations](https://httpwg.org/specs/rfc8297.html#security-considerations)
- [Chrome only implemented support on HTTP/2 or above](https://bugs.chromium.org/p/chromium/issues/detail?id=671310)
- [Safari only implemented support on HTTP/2 or above](https://github.com/WebKit/WebKit/pull/10836)
- [Cloudflare only implemented support on HTTP/2 or above](https://developers.cloudflare.com/cache/advanced-configuration/early-hints/#:~:text=Early%20Hints%20is%20currently%20only%20supported%20over%20HTTP/2%20and%20HTTP/3.)
- [Fastly only implemented support on HTTP/2 or above](https://www.fastly.com/blog/beyond-server-push-experimenting-with-the-103-early-hints-status-code#is-it-safe)
- [Apache only implemented support on HTTP/2 or above](https://httpd.apache.org/docs/2.4/mod/mod_http2.html#h2earlyhints)

In fact the only HTTP/1.1 implementation I am aware of is [Firefox](https://bugzilla.mozilla.org/show_bug.cgi?id=1813035#c6), but this still has not been enabled by default.

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