also run tests on net7 (#1010)

Author: SimonCropp

src/Polly.Specs/Polly.Specs.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net6.0</TargetFrameworks>
+    <TargetFrameworks>net6.0;net7.0</TargetFrameworks>
     <TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('Windows'))">$(TargetFrameworks);net481</TargetFrameworks>
     <DisableImplicitNuGetFallbackFolder>true</DisableImplicitNuGetFallbackFolder>
     <Nullable>enable</Nullable>