Update version to 2.45.0-pre1 (on v2.45.x) (#1675)

Author: jtattermusch

build/version.props

@@ -2,7 +2,7 @@
   <PropertyGroup>
 
     <!-- package version of grpc-dotnet -->
-    <GrpcDotnetVersion>2.45.0-dev</GrpcDotnetVersion>
+    <GrpcDotnetVersion>2.45.0-pre1</GrpcDotnetVersion>
 
     <!-- assembly version of grpc-dotnet -->
     <GrpcDotnetAssemblyVersion>2.0.0.0</GrpcDotnetAssemblyVersion>