Replace ExportSrtpKeyingMaterial with AppendSrtpKeyingMaterial

The latter has internal computation of key sizes, and will use
the Buffer argument in a normal manner, making the use of
Buffer::CreateUninitializedWithSize() unnecessary.

ExportSrtpKeyingMaterial is deprecated.

Bug: webrtc:475583775
Change-Id: I56a065bb90c0f2a2d9dfd4a4d05773cf7e887763
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/442100
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46669}