move tests from net472 to 481 (#1011)

Author: SimonCropp

src/Polly.Specs/Polly.Specs.csproj

@@ -2,7 +2,7 @@
 
   <PropertyGroup>
     <TargetFrameworks>net6.0</TargetFrameworks>
-    <TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('Windows'))">$(TargetFrameworks);net472</TargetFrameworks>
+    <TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('Windows'))">$(TargetFrameworks);net481</TargetFrameworks>
     <DisableImplicitNuGetFallbackFolder>true</DisableImplicitNuGetFallbackFolder>
     <Nullable>enable</Nullable>
   </PropertyGroup>