Regenerate all APIs using protoc 3.13 and Grpc.Tools 2.31

Note that the change in gRPC generation is *just possibly* breaking,
as it changes a constructor parameter type (to a base type). This is
far less likely to break anyone than changing a *method* parameter,
as method group conversions aren't involved. It also removes the
dependency on Grpc.Core from the generated code.

apis/Google.Analytics.Admin.V1Alpha/Google.Analytics.Admin.V1Alpha/AnalyticsAdminGrpc.cs

@@ -1042,7 +1042,7 @@ public partial class AnalyticsAdminServiceClient : grpc::ClientBase<AnalyticsAdm
     {
       /// <summary>Creates a new client for AnalyticsAdminService</summary>
       /// <param name="channel">The channel to use to make remote calls.</param>
-      public AnalyticsAdminServiceClient(grpc::Channel channel) : base(channel)
+      public AnalyticsAdminServiceClient(grpc::ChannelBase channel) : base(channel)
       {
       }
       /// <summary>Creates a new client for AnalyticsAdminService that uses a custom <c>CallInvoker</c>.</summary>