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_transceiver_impl.h

@@ -16,8 +16,8 @@
 #include <string>
 #include <vector>
 
+#include "absl/types/optional.h"
 #include "api/array_view.h"
-#include "api/optional.h"
 #include "modules/rtp_rtcp/source/rtcp_packet/common_header.h"
 #include "modules/rtp_rtcp/source/rtcp_packet/dlrr.h"
 #include "modules/rtp_rtcp/source/rtcp_packet/remb.h"
@@ -91,7 +91,7 @@ class RtcpTransceiverImpl {
   const RtcpTransceiverConfig config_;
 
   bool ready_to_send_;
-  rtc::Optional<rtcp::Remb> remb_;
+  absl::optional<rtcp::Remb> remb_;
   // TODO(danilchap): Remove entries from remote_senders_ that are no longer
   // needed.
   std::map<uint32_t, RemoteSenderState> remote_senders_;