Merged PR 750956: [Blob L3 GC] Enable deleting old untracked containers by default

[Blob L3 GC] Enable deleting old untracked containers by default. This has been running successfully for a couple of days in the cbrmtest-centralus cache, which is the main cache where we've been doing experiments. Enabling for all other caches. This keeps the option of disabling via config.

Author: JuanCarlosGI

Public/Src/Cache/LifetimeManager/Library/BlobQuotaKeeperConfig.cs

@@ -26,9 +26,8 @@ public class BlobQuotaKeeperConfig
 
         /// <summary>
         /// Specifies how old an untracked namespace should be before we delete it. Set to null to disable the feature.
-        /// TODO: Set a default of 2 days once we have rolled this out.
         /// </summary>
-        public TimeSpan? UntrackedNamespaceDeletionThreshold { get; set; } = null;
+        public TimeSpan? UntrackedNamespaceDeletionThreshold { get; set; } = TimeSpan.FromDays(2);
 
         /// <summary>
         /// Retry policy for Azure Storage client.