Build samples with .csproj files instead of .sln files

- Some of the samples do not have .sln files, so there were some failures
  while trying to install NuGet packages. Let's run the nuget restore and
  msbuild commands with the .csproj files instead.

- The WebApplication sample doesn't have a .csproj or .sln file, so
  we will be unable to run it in CI.

.github/workflows/test-net-framework-samples.yml

@@ -84,7 +84,6 @@ jobs:
               'OpticalCharacterRecognition/AddTextToImage/',
               'Other/MemoryFileSystem/',
               'Other/StreamIO/',
-              'Other/WebApplication/',
               'Printing/PrintPDF/',
               'Printing/PrintPDFGUI/',
               'Security/AddRegexRedaction/',
@@ -123,11 +122,11 @@ jobs:
             run: |
               sample_name=$(basename "$PWD")
               echo "Installing NuGet packages..."
-              nuget.exe restore $sample_name.sln
+              nuget.exe restore $sample_name.csproj
 
           - name: Build samples
             working-directory: ${{ matrix.dir }}
             run: |
               sample_name=$(basename "$PWD")
               echo "Building $sample_name sample..."
-              msbuild.exe $sample_name.sln //p:Configuration=Release
+              msbuild.exe $sample_name.csproj //p:Configuration=Release