Bump to WebRTC M131 release

Ongoing fixes and improvements, transient suppressor is gone. Also,
dropping isac because it doesn't seem to be useful, and is just build
system deadweight now.

Upstream references:

  Version: 131.0.6778.200
  WebRTC: 79aff54b0fa9238ce3518dd9eaf9610cd6f22e82
  Chromium: 2a19506ad24af755f2a215a4c61f775393e0db42

webrtc/modules/audio_processing/aec3/echo_canceller3.h

@@ -15,9 +15,9 @@
 
 #include <atomic>
 #include <memory>
+#include <optional>
 #include <vector>
 
-#include "absl/types/optional.h"
 #include "api/array_view.h"
 #include "api/audio/echo_canceller3_config.h"
 #include "api/audio/echo_control.h"
@@ -88,12 +88,11 @@ class Aec3RenderQueueItemVerifier {
 // AnalyzeRender call which can be called concurrently with the other methods.
 class EchoCanceller3 : public EchoControl {
  public:
-  EchoCanceller3(
-      const EchoCanceller3Config& config,
-      const absl::optional<EchoCanceller3Config>& multichannel_config,
-      int sample_rate_hz,
-      size_t num_render_channels,
-      size_t num_capture_channels);
+  EchoCanceller3(const EchoCanceller3Config& config,
+                 const std::optional<EchoCanceller3Config>& multichannel_config,
+                 int sample_rate_hz,
+                 size_t num_render_channels,
+                 size_t num_capture_channels);
 
   ~EchoCanceller3() override;