dcsctp: Correct outstanding data calculation

This commit fixes the calculation of "outstanding data" (flightsize) to
correctly handle scenarios involving SACK Gap Ack Blocks, ensuring
compliance with RFC 9260.

Previously, the outstanding data count may have been incorrect when a
SACK acknowledged non-sequential TSNs. For example, in the unit test
OutstandingDataTest.AcksAndNacksWithGapAckBlocks, if a sender transmits
TSN=10 and TSN=11, and receives a SACK with Cumulative TSN Ack=9 and a
Gap Ack Block for TSN=11 (2:2), TSN=10 must remain counted as
outstanding data, while TSN=11 must be removed.

Per RFC 9260 Section 1.3, "Outstanding Data" is defined as "The total
size of the DATA chunks associated with outstanding TSNs." An
"Outstanding TSN" is a TSN that "has been sent by the endpoint but for
which it has not yet received an acknowledgement."

Bug: None
Change-Id: Ia7352942bd17c2ed20e70f4cc4e8803b57799365
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/446060
Reviewed-by: Evan Shrubsole <eshr@webrtc.org>
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46814}