Remove unsupported versions of .NET from README.

.NET Core 2.2 and 3.0 are no longer supported and should not be in the examples.

All versions were changed to pull in the latest revision as people might copy this without
checking all accepted version formats and getting the latest revision is good security practice.

README.md

@@ -37,7 +37,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        dotnet: [ '2.2.103', '3.0', '3.1.x' ]
+        dotnet: [ '2.1.x', '3.1.x', '5.0.x' ]
     name: Dotnet ${{ matrix.dotnet }} sample
     steps:
       - uses: actions/checkout@v2
@@ -59,7 +59,7 @@ jobs:
       - name: Setup dotnet
         uses: actions/setup-dotnet@v1
         with:
-          dotnet-version: '2.2.103'
+          dotnet-version: '2.1.x'
       - name: Setup dotnet
         uses: actions/setup-dotnet@v1
         with:
@@ -113,7 +113,7 @@ build:
     - uses: actions/checkout@main
     - uses: actions/setup-dotnet@v1
       with:
-        dotnet-version: '3.1.100' # SDK Version to use.
+        dotnet-version: '3.1.x' # SDK Version to use.
 ```
 
 # License