Remove .NET 5 from targets (#434)

* Remove .NET 5 from targets

No need of targeting it explicitly, since most of the consumers updated or can use `.netstandard2.0` dll.

* Update build.yml

* Update Dockerfile

.devcontainer/Dockerfile

@@ -1,12 +1,9 @@
 # See here for image contents: https://github.com/microsoft/vscode-dev-containers/tree/v0.192.0/containers/dotnet/.devcontainer/base.Dockerfile
 
 # [Choice] .NET version: 5.0, 3.1, 2.1
-ARG VARIANT="5.0"
+ARG VARIANT="6.0"
 FROM mcr.microsoft.com/vscode/devcontainers/dotnet:0-${VARIANT}
 
-# Add .NET Core 3.1 runtime.
-COPY --from=mcr.microsoft.com/dotnet/core/aspnet:3.1-buster-slim /usr/share/dotnet /usr/share/dotnet
-
 # [Optional] Uncomment this section to install additional OS packages.
 # RUN apt-get update && export DEBIAN_FRONTEND=noninteractive \
-#    && apt-get -y install --no-install-recommends servicefabricsdkcommon
+#    && apt-get -y install --no-install-recommends servicefabricsdkcommon

.github/workflows/build.yml

@@ -24,12 +24,6 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - uses: actions/setup-dotnet@v1
-      with:
-        dotnet-version: '3.1.x'
-    - uses: actions/setup-dotnet@v1
-      with:
-        dotnet-version: '5.0.x'
     - uses: actions/setup-dotnet@v1
       with:
         dotnet-version: '6.0.x'

Directory.Build.props

@@ -1,7 +1,7 @@
 <Project>
   <Import Project="$(MSBuildThisFileDirectory)\Dependencies.props" />
   <PropertyGroup Label="Target Platforms" >
-    <NetCoreVersions>net5.0;net6.0</NetCoreVersions>
+    <NetCoreVersions>net6.0</NetCoreVersions>
     <NetStandardVersions>netstandard2.0</NetStandardVersions>
     <LibraryTargetFrameworks>$(NetCoreVersions);$(NetStandardVersions)</LibraryTargetFrameworks>
     <ExecutableTargetFrameworks>$(NetCoreVersions)</ExecutableTargetFrameworks>