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/include/rtp_payload_registry.h

@@ -14,8 +14,8 @@
 #include <map>
 #include <set>
 
+#include "absl/types/optional.h"
 #include "api/audio_codecs/audio_format.h"
-#include "api/optional.h"
 #include "api/video_codecs/video_codec.h"
 #include "modules/rtp_rtcp/source/rtp_utility.h"
 #include "rtc_base/criticalsection.h"
@@ -42,7 +42,7 @@ class RTPPayloadRegistry {
 
   int GetPayloadTypeFrequency(uint8_t payload_type) const;
 
-  rtc::Optional<RtpUtility::Payload> PayloadTypeToPayload(
+  absl::optional<RtpUtility::Payload> PayloadTypeToPayload(
       uint8_t payload_type) const;
 
   void ResetLastReceivedPayloadTypes() {