Update version to 2.33.1-pre1 on the release branch (#1086)

Author: jtattermusch

build/version.props

@@ -2,13 +2,13 @@
   <PropertyGroup>
 
     <!-- package version of grpc-dotnet -->
-    <GrpcDotnetVersion>2.31.0-dev</GrpcDotnetVersion>
+    <GrpcDotnetVersion>2.33.1-pre1</GrpcDotnetVersion>
 
     <!-- assembly version of grpc-dotnet -->
     <GrpcDotnetAssemblyVersion>2.0.0.0</GrpcDotnetAssemblyVersion>
 
     <!-- file version of all grpc-dotnet -->
-    <GrpcDotnetAssemblyFileVersion>2.31.0.0</GrpcDotnetAssemblyFileVersion>
+    <GrpcDotnetAssemblyFileVersion>2.33.1.0</GrpcDotnetAssemblyFileVersion>
 
   </PropertyGroup>
 </Project>