Disable OmniSharp by default for C# files (#44427)

In preparation for https://github.com/zed-extensions/csharp/pull/11. Do
not merge before that PR is published.

Release Notes:

- Added support for Roslyn in C# files. Roslyn will now be the default
language server for C#

assets/settings/default.json

@@ -1810,6 +1810,9 @@
         "allowed": false
       }
     },
+    "CSharp": {
+      "language_servers": ["roslyn", "!omnisharp", "..."]
+    },
     "CSS": {
       "prettier": {
         "allowed": true