Migrate absl::optional to std::optional

Bug: webrtc:342905193
No-Try: True
Change-Id: Icc968be43b8830038ea9a1f5f604307220457807
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/361021
Auto-Submit: Florent Castelli <orphis@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Florent Castelli <orphis@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42911}
diff --git a/modules/audio_coding/neteq/audio_decoder_unittest.cc b/modules/audio_coding/neteq/audio_decoder_unittest.cc
index a39b46b..ddcf020 100644
--- a/modules/audio_coding/neteq/audio_decoder_unittest.cc
+++ b/modules/audio_coding/neteq/audio_decoder_unittest.cc
@@ -419,7 +419,7 @@
 
 namespace {
 int SetAndGetTargetBitrate(AudioEncoder* audio_encoder, int rate) {
-  audio_encoder->OnReceivedUplinkBandwidth(rate, absl::nullopt);
+  audio_encoder->OnReceivedUplinkBandwidth(rate, std::nullopt);
   return audio_encoder->GetTargetBitrate();
 }
 void TestSetAndGetTargetBitratesWithFixedCodec(AudioEncoder* audio_encoder,