Dropped netcore 2.1; upgraded build vers

Author: jburnett

.github/workflows/dotnet-core.yml

@@ -4,7 +4,7 @@ on:
   push:
     branches: [ master, dev ]
   pull_request:
-    branches: [ dev ]
+    branches: [ master ]
 
 jobs:
   build:
@@ -16,15 +16,11 @@ jobs:
     - name: Setup .NET 5.0
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 5.0.202
+        dotnet-version: 5.0.401
     - name: Setup .NET Core 3.1
       uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: 3.1.407
-    - name: Setup .NET Core 2.1
-      uses: actions/setup-dotnet@v1
-      with:
-        dotnet-version: 2.1.814
+        dotnet-version: 3.1.413
     - name: Install dependencies
       run: dotnet restore
     - name: Build