Remove remove support for .net 6 and 7

Author: damianh

libs/src.props

@@ -45,14 +45,6 @@
     <ContinuousIntegrationBuild>true</ContinuousIntegrationBuild>
   </PropertyGroup>
 
-  <PropertyGroup Condition=" '$(TargetFramework)' == 'net6.0'">
-    <FrameworkVersion>6.0.0</FrameworkVersion>
-  </PropertyGroup>
-
-  <PropertyGroup Condition=" '$(TargetFramework)' == 'net7.0'">
-    <FrameworkVersion>7.0.0</FrameworkVersion>
-  </PropertyGroup>
-
   <PropertyGroup Condition=" '$(TargetFramework)' == 'net8.0'">
     <FrameworkVersion>8.0.0</FrameworkVersion>
   </PropertyGroup>