set version prefix to 3.0.0

Author: KoditkarVedant

Src/Notion.Client/Notion.Client.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <VersionPrefix>2.2.3-preview</VersionPrefix>
+    <VersionPrefix>3.0.0-preview</VersionPrefix>
     <TargetFramework>netstandard2.0</TargetFramework>
     <LangVersion>7.3</LangVersion>