Replace rtc::Optional with absl::optional in api

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

This CL generated by running script with parameter 'api'
Then undo changes to optional target itself and optional_unittests

find $@ -type f \( -name \*.h -o -name \*.cc \) \
-exec sed -i 's|rtc::Optional|absl::optional|g' {} \+ \
-exec sed -i 's|rtc::nullopt|absl::nullopt|g' {} \+ \
-exec sed -i 's|#include "api/optional.h"|#include "absl/types/optional.h"|' {} \+

find $@ -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: I44093da213369d6a502e33792c694f620f53b779
Reviewed-on: https://webrtc-review.googlesource.com/84621
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23707}

api/rtpreceiverinterface.h

@@ -57,8 +57,8 @@ class RtpSource {
   // The source can be either a contributing source or a synchronization source.
   RtpSourceType source_type() const { return source_type_; }
 
-  rtc::Optional<uint8_t> audio_level() const { return audio_level_; }
-  void set_audio_level(const rtc::Optional<uint8_t>& level) {
+  absl::optional<uint8_t> audio_level() const { return audio_level_; }
+  void set_audio_level(const absl::optional<uint8_t>& level) {
     audio_level_ = level;
   }
 
@@ -71,7 +71,7 @@ class RtpSource {
   int64_t timestamp_ms_;
   uint32_t source_id_;
   RtpSourceType source_type_;
-  rtc::Optional<uint8_t> audio_level_;
+  absl::optional<uint8_t> audio_level_;
 };
 
 class RtpReceiverObserverInterface {