HTTP/2: switch to RFC 9218 priorities

HTTP/2: switch to RFC 9218 priorities

Advertise SETTINGS_NO_RFC7540_PRIORITIES (0x9) in the initial SETTINGS
frame and order the output by the RFC 9218 effective priority instead of
the RFC 7540 dependency tree, which is removed in the same step.
Placing the switch last keeps it self-contained and easy to revert.

RFC 9218, Section 2.1: a server can send SETTINGS_NO_RFC7540_PRIORITIES
with a value of 1 to indicate that it will ignore RFC 7540 priority
signals.  A received value other than 0 or 1 is a connection error.

Frames are ordered by urgency first, then non-incremental ahead of
incremental, then by stream id, which keeps streams of equal priority
in the order their requests arrived (RFC 8941, Section 4.2).  The same
order is applied to ngx_http_v2_waiting_queue().

The RFC 7540 priority scheme is removed: PRIORITY frames are validated
for framing errors and then ignored, the priority fields of a HEADERS
frame are skipped, and the dependency tree (node parent/children/weight,
the dependencies queue, ngx_http_v2_set_dependency(),
ngx_http_v2_node_children_update() and the per-connection
priority_limit) is gone.  RFC 9113, Sections 5.3.1 and 5.3.2, deprecate
stream prioritization: it proved complex, was implemented unevenly and
widely ignored, and its protocol elements are retained for wire
compatibility only.

With the tree gone, a buffered PRIORITY_UPDATE node no longer needs to
be attached to it, so the set_dependency() call from the buffering path
falls away, leaving the node parked in h2c->closed on its own.

Assisted-by: opencode:claude-opus-4.8