Downgrade dotnet-format and move .csproj

Use an older version of dotnet-format because the newer version shows
absolute paths which breaks the 'dotnet-format-problem-matcher'. [1]

Also move the dummy .csproj file to the root of the repo so that
annotations will be created. [2]

[1] xt0rted/dotnet-format-problem-matcher#206
[2] xt0rted/dotnet-format-problem-matcher#91

Author: Histalek

.github/workflows/dotnet.yml

@@ -31,7 +31,7 @@ jobs:
         uses: xt0rted/dotnet-format-problem-matcher@v1
 
       - name: Setup dotnet-format tool
-        run: dotnet tool install -g dotnet-format
+        run: dotnet tool install -g dotnet-format --version 5.0.\*
 
       - name: Run dotnet format
-        run: dotnet format code/tttreborn-ci.csproj --check
+        run: dotnet format --check