Update actions/cache action to v4

Author: renovate[bot]

.github/workflows/main.yml

@@ -30,7 +30,7 @@ jobs:
         with:
           dotnet-version: '8.0.x'
       - name: NuGet Cache
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           path: ${{ env.NUGET_PACKAGES }}
           key: ${{ runner.os }}.nuget.${{ hashFiles('**/*.fsproj') }}