update workflow to use actions/cache@v4

Author: nitz

.github/workflows/dotnet.yaml

@@ -43,7 +43,7 @@ jobs:
 
     - name: Cache .NET Toolchains
       id: cache-dotnet-toolchain
-      uses: actions/cache@v2
+      uses: actions/cache@v4
       with:
         path: '%LocalAppData%\Microsoft\dotnet\'
         key: 'dotnet-toolchain'