.NET: Downgrade Microsoft.CodeAnalysis.CSharp to 4.8.0 for compat with SDK 8.0.1xy

And for CI, set up .NET SDK 8.0.100 explicitly to test our min supported version.

.github/workflows/linux_builds.yml

@@ -140,6 +140,18 @@ jobs:
           python-version: 3.8
           scons-version: 4.0
 
+      - name: Force remove preinstalled .NET SDKs
+        if: matrix.build-mono
+        run: |
+          sudo rm -rf /usr/share/dotnet/sdk/*
+
+      - name: Setup older .NET SDK as baseline
+        if: matrix.build-mono
+        uses: actions/setup-dotnet@v4
+        with:
+          # Targeting the oldest version we want to support to ensure it still builds.
+          dotnet-version: '8.0.100'
+
       - name: Compilation
         uses: ./.github/actions/godot-build
         with:
@@ -163,6 +175,7 @@ jobs:
       - name: Build .NET solutions
         if: matrix.build-mono
         run: |
+          dotnet --info
           ./modules/mono/build_scripts/build_assemblies.py --godot-output-dir=./bin --godot-platform=linuxbsd
 
       - name: Prepare artifact