MINOR: quic: rename pacing_rate cb to pacing_inter

MINOR: quic: rename pacing_rate cb to pacing_inter

Rename one of the congestion algorithms pacing callback from pacing_rate
to pacing_inter. This better reflects that this function returns a delay
(in nanoseconds) which should be applied between each packet emission to
fill the congestion window with a perfectly smoothed emission.

This should be backported up to 3.1.

(cherry picked from commit 7c0820892f47642cfa27e49aa1218dfaf640e3b2)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>