http2: support SETTINGS_NO_RFC7540_PRIORITIES in SETTINGS frame

To make sure that clients do not unnecessarily send RFC 7540 priority
signals when it would be treated as a no-op, this change makes it so
that our server always sends SETTINGS_NO_RFC7540_PRIORITIES in our
SETTINGS frame when our write scheduler is set to anything other than
the RFC 7540 write scheduler.

For golang/go#75500

Change-Id: I7a54251022087319999deda7efb663f8b251aa95
Reviewed-on: https://go-review.googlesource.com/c/net/+/729141
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>