Set memory cache logger field as readonly (#37289)

Author: lindexi

src/libraries/Microsoft.Extensions.Caching.Memory/src/MemoryCache.cs

@@ -23,7 +23,7 @@ public class MemoryCache : IMemoryCache
  private readonly ConcurrentDictionary<object, CacheEntry> _entries;
  private long _cacheSize;
  private bool _disposed;
- private ILogger _logger;
+ private readonly ILogger _logger;
 
  // We store the delegates locally to prevent allocations
  // every time a new CacheEntry is created.