Replace rtc::Optional with absl::optional in modules/rtp_rtcp

This is a no-op change because rtc::Optional is an alias to absl::optional

This CL generated using script:
#!/bin/bash
dir=modules/rtp_rtcp
find $dir -type f \( -name \*.h -o -name \*.cc \) \
-exec sed -i 's|rtc::Optional|absl::optional|' {} \+ \
-exec sed -i 's|rtc::nullopt|absl::nullopt|' {} \+ \
-exec sed -i 's|#include "api/optional.h"|#include "absl/types/optional.h"|' {} \+

find $dir -type f -name BUILD.gn \
-exec sed -r -i 's|"(../)*api:optional"|"//third_party/abseil-cpp/absl/types:optional"|' {} \+;

git cl format

Bug: webrtc:9078
Change-Id: Ife720849709959046329c1c9faa3f31aa13274dc
Reviewed-on: https://webrtc-review.googlesource.com/83584
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23624}

modules/rtp_rtcp/source/rtcp_receiver.cc

@@ -72,7 +72,7 @@ struct RTCPReceiver::PacketInformation {
   int64_t rtt_ms = 0;
   uint32_t receiver_estimated_max_bitrate_bps = 0;
   std::unique_ptr<rtcp::TransportFeedback> transport_feedback;
-  rtc::Optional<VideoBitrateAllocation> target_bitrate_allocation;
+  absl::optional<VideoBitrateAllocation> target_bitrate_allocation;
 };
 
 // Structure for handing TMMBR and TMMBN rtcp messages (RFC5104, section 3.5.4).