Switch back to .NET Core 3.1 because .NET 5 is neither supported not yet released (#6096)

frameworks/CSharp/genhttp/Benchmarks/Benchmarks.csproj

@@ -2,8 +2,7 @@
   
   <PropertyGroup>
     
-    <TargetFramework>net5.0</TargetFramework>
-
+    <TargetFramework>netcoreapp3.1</TargetFramework>
     <LangVersion>8.0</LangVersion>
     
     <AssemblyTitle>GenHTTP Benchmarks</AssemblyTitle>

frameworks/CSharp/genhttp/genhttp.dockerfile

@@ -1,4 +1,4 @@
-FROM mcr.microsoft.com/dotnet/sdk:5.0-alpine AS build
+FROM mcr.microsoft.com/dotnet/core/sdk:3.1-alpine AS build
 WORKDIR /source
 
 # copy csproj and restore as distinct layers
@@ -10,7 +10,7 @@ COPY Benchmarks/ .
 RUN dotnet publish -c release -o /app -r linux-musl-x64 --self-contained true --no-restore /p:PublishTrimmed=true /p:PublishReadyToRun=true
 
 # final stage/image
-FROM mcr.microsoft.com/dotnet/runtime-deps:5.0-alpine
+FROM mcr.microsoft.com/dotnet/core/runtime-deps:3.1-alpine
 WORKDIR /app
 COPY --from=build /app .