Revert adding net 9.0

Author: IgorBuchelnikov

src/ObservableComputations/ObservableComputations.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net461;net48;netcoreapp2.1;netcoreapp3.1;net5.0;net9.0;netstandard2.0</TargetFrameworks>
+    <TargetFrameworks>net461;net48;netcoreapp2.1;netcoreapp3.1;net5.0;netstandard2.0</TargetFrameworks>
     <SignAssembly>true</SignAssembly>
     <Platforms>AnyCPU;x86</Platforms>
     <LangVersion>7.1</LangVersion>