chore(ci): update actions/cache action to v6

Author: renovate[bot]

.github/workflows/build-dotnet-fast.yml

@@ -88,7 +88,7 @@ jobs:
           global-json-file: ./global.json
 
       - name: NuGet cache
-        uses: actions/cache@27d5ce7f107fe9357f9df03efb73ab90386fccae # v5.0.5
+        uses: actions/cache@2c8a9bd7457de244a408f35966fab2fb45fda9c8 # v6.0.0
         with:
           path: ~/.nuget/packages
           key: ${{ runner.os }}-nuget-${{ hashFiles('**/*.csproj') }}