Fix CoreCLR serialization and deprecation errors

- Add [Serializable] to Monitor and ParameterOverride base classes to
  satisfy UAC1002 serialization hierarchy checks
- Add [NonSerialized] to PostProcessLayer.SerializedBundleRef.bundle to
  suppress UAC1001 for intentionally non-serialized field
- Replace FindFirstObjectByType with FindAnyObjectByType to fix CS0618

Author: ApoorvaJ

TestProjects/PostProcessing_Tests/Assets/CommonAssets/Scripts/PostProcessingGraphicsTests.cs

@@ -20,7 +20,7 @@ public IEnumerator Run(GraphicsTestCase testCase)
         var camera = GameObject.FindGameObjectWithTag("MainCamera").GetComponent<Camera>();
 
 #if UNITY_2020_3_OR_NEWER
-        var settings = Object.FindFirstObjectByType<PostProcessingGraphicsTestSettings>();
+        var settings = Object.FindAnyObjectByType<PostProcessingGraphicsTestSettings>();
 #else
         var settings = Object.FindObjectOfType<PostProcessingGraphicsTestSettings>();
 #endif

com.unity.postprocessing/PostProcessing/Runtime/Monitors/Monitor.cs

@@ -1,3 +1,5 @@
+using System;
+
 namespace UnityEngine.Rendering.PostProcessing
 {
     /// <summary>
@@ -29,6 +31,7 @@ public enum MonitorType
     /// <summary>
     /// The base class for all debug monitors.
     /// </summary>
+    [Serializable]
     public abstract class Monitor
     {
         /// <summary>

com.unity.postprocessing/PostProcessing/Runtime/ParameterOverride.cs

@@ -6,6 +6,7 @@ namespace UnityEngine.Rendering.PostProcessing
     /// The base abstract class for all parameter override types.
     /// </summary>
     /// <seealso cref="ParameterOverride{T}"/>
+    [Serializable]
     public abstract class ParameterOverride
     {
         /// <summary>

com.unity.postprocessing/PostProcessing/Runtime/PostProcessLayer.cs

@@ -157,6 +157,7 @@ public sealed class SerializedBundleRef
             /// <summary>
             /// A reference to the bundle itself.
             /// </summary>
+            [NonSerialized]
             public PostProcessBundle bundle; // Not serialized, is set/reset when deserialization kicks in
         }