[ObjC] Change default sdpSemantics to NotSpecified.

The default value of sdpSemantics is about to change from PlanB to
UnifiedPlan. In order not to cause subtle bugs by applications that
depend on the default value being PlanB, we are temporarily making the
default NotSpecified. Constructing with NotSpecified causes the C++
layer to crash (https://webrtc-review.googlesource.com/c/src/+/242968).
This is in accordance to the publically announced plans:
https://groups.google.com/u/1/g/discuss-webrtc/c/SdoVP02eUIk

Bug: webrtc:11121
Change-Id: Idbb8fd0f5c224311cf1f25ac2832800124ed14d4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/246060
Reviewed-by: Peter Hanspers <peterhanspers@webrtc.org>
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35678}

sdk/objc/api/peerconnection/RTCConfiguration.mm

@@ -68,7 +68,7 @@
 - (instancetype)init {
   // Copy defaults.
   webrtc::PeerConnectionInterface::RTCConfiguration config;
-  config.sdp_semantics = webrtc::SdpSemantics::kPlanB_DEPRECATED;
+  config.sdp_semantics = webrtc::SdpSemantics::kNotSpecified;
   return [self initWithNativeConfiguration:config];
 }
 
@@ -525,6 +525,8 @@
       return webrtc::SdpSemantics::kPlanB_DEPRECATED;
     case RTCSdpSemanticsUnifiedPlan:
       return webrtc::SdpSemantics::kUnifiedPlan;
+    case RTCSdpSemanticsNotSpecified:
+      return webrtc::SdpSemantics::kNotSpecified;
   }
 }
 
@@ -535,8 +537,7 @@
     case webrtc::SdpSemantics::kUnifiedPlan:
       return RTCSdpSemanticsUnifiedPlan;
     case webrtc::SdpSemantics::kNotSpecified:
-      RTC_DCHECK_NOTREACHED();
-      return RTCSdpSemanticsUnifiedPlan;
+      return RTCSdpSemanticsNotSpecified;
   }
 }
 
@@ -546,6 +547,8 @@
       return @"PLAN_B";
     case RTCSdpSemanticsUnifiedPlan:
       return @"UNIFIED_PLAN";
+    case RTCSdpSemanticsNotSpecified:
+      return @"NOT_SPECIFIED";
   }
 }