Small cleanup for Lock in Mono

- Used the project property instead of disabling the compiler warning. Probably a bit cleaner, and would enable using Lock in other libraries to try it out.
- Undid an unnecessary change from dotnet#87672

src/mono/System.Private.CoreLib/System.Private.CoreLib.csproj

@@ -13,6 +13,9 @@
     <Platforms>x64;x86;arm;armv6;arm64;riscv64;s390x;wasm;ppc64le</Platforms>
 
     <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
+
+    <!-- CA2252: Opt in to preview features before using them (Lock) -->
+    <EnablePreviewFeatures>true</EnablePreviewFeatures>
   </PropertyGroup>
 
   <!-- Note that various places in SPCL depend on this resource name i.e. TplEventSource -->

src/mono/System.Private.CoreLib/src/System/Threading/Monitor.Mono.cs

@@ -145,9 +145,7 @@ private static void ReliableEnterTimeout(object obj, int timeout, ref bool lockT
             try_enter_with_atomic_var(obj, timeout, true, ref lockTaken);
         }
 
-#pragma warning disable CA2252 // Opt in to preview features before using them (Lock)
         public static long LockContentionCount => Monitor_get_lock_contention_count() + Lock.ContentionCount;
-#pragma warning restore CA2252
 
         [MethodImplAttribute(MethodImplOptions.InternalCall)]
         private static extern long Monitor_get_lock_contention_count();

src/mono/System.Private.CoreLib/src/System/Threading/TimerQueue.Browser.Mono.cs

@@ -16,7 +16,7 @@ namespace System.Threading
     //
     internal partial class TimerQueue
     {
-        public static long TickCount64 => Environment.TickCount64;
+        private static long TickCount64 => Environment.TickCount64;
         private static List<TimerQueue>? s_scheduledTimers;
         private static List<TimerQueue>? s_scheduledTimersToFire;
         private static long s_shortestDueTimeMs = long.MaxValue;