Gen v1.21.0 (#603)

* gen v1.21.0

* update version converter

* bump version to 5

* remove support of netstandard2.0

* update test dependency

.github/workflows/buildtest.yaml

@@ -44,10 +44,6 @@ jobs:
       - uses: actions/checkout@v1
         with:
           fetch-depth: 0
-      - name: Setup dotnet SDK 2.1
-        uses: actions/setup-dotnet@v1
-        with:
-          dotnet-version: '2.1.x'
       - name: Setup dotnet SDK 3.1
         uses: actions/setup-dotnet@v1
         with:

.github/workflows/nuget.yaml

@@ -13,10 +13,6 @@ jobs:
     - uses: actions/checkout@v1
       with:
         fetch-depth: 0
-    - name: Setup dotnet SDK 2.1
-      uses: actions/setup-dotnet@v1
-      with:
-        dotnet-version: '2.1.x'
     - name: Setup dotnet SDK 3.1
       uses: actions/setup-dotnet@v1
       with: