Revert langversion upgrade.

Author: timcassell

build/common.props

@@ -33,7 +33,7 @@
   </PropertyGroup>
 
   <PropertyGroup Condition=" '$(IsVisualBasic)' != 'true' AND '$(IsFsharp)' != 'true' ">
-    <LangVersion>11</LangVersion>
+    <LangVersion>9.0</LangVersion>
 
     <Major>0</Major>
     <Minor>13</Minor>

src/BenchmarkDotNet/Code/DeclarationsProvider.cs

@@ -94,10 +94,7 @@ public override string OverheadImplementation
                 {
                     return $"return default({type.GetCorrectCSharpTypeName()});";
                 }
-                return $"""
-                    System.Runtime.CompilerServices.Unsafe.SkipInit(out {type.GetCorrectCSharpTypeName()} value);
-                    return value;
-                    """;
+                return $"System.Runtime.CompilerServices.Unsafe.SkipInit(out {type.GetCorrectCSharpTypeName()} value);\nreturn value;";
             }
         }