also benchmark on net7 (#1009)

Author: SimonCropp

src/Polly.Benchmarks/Polly.Benchmarks.csproj

@@ -2,7 +2,7 @@
   <PropertyGroup>
     <IsPackable>false</IsPackable>
     <OutputType>Exe</OutputType>
-    <TargetFramework>net6.0</TargetFramework>
+    <TargetFrameworks>net6.0;net7.0</TargetFrameworks>
     <Nullable>enable</Nullable>
   </PropertyGroup>
   <ItemGroup>