GravityShader: verbatim string, not a raw string literal (Unity's C# 9)

Commit 13c7a087 and the merge a1b54bea do not build: error CS8773, raw string literals need C# 11.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Assets/Scripts/ServerShared/GravityShader.cs

@@ -16,8 +16,7 @@ public static class GravityShader
         .Replace("@TAIL@", GravityField.TailOffset.ToString("R", CultureInfo.InvariantCulture)))
         .Replace("\r\n", "\n");
 
-    private const string Template = """
-#ifndef AETHERIA_GRAVITY_INCLUDED
+    private const string Template = @"#ifndef AETHERIA_GRAVITY_INCLUDED
 #define AETHERIA_GRAVITY_INCLUDED
 
 // GENERATED by ServerShared/GravityShader.cs from GravityField; do not edit (GravityShaderTests fails on any difference).
@@ -60,6 +59,5 @@ float gravityHeight(float2 p)
 }
 
 #endif
-
-""";
+";
 }