provide a net9 build target

[bitfaster]

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• Use new System.Threading.Lock type in ConcurrentLfuCore. Benchmark shows Lock.TryEnter is faster.
• POC of TryGetAlternateCache for string keys, see https://github.com/dotnet/runtime/issues/27229
  • The lookup is accessed by the GetAlternateLookup extension method
  • Note, IAlternateEqualityComparer can create the original T from the alternate, so we can allocate strings only for cache miss and pass into event handlers etc.
• TODO: net9 specific unit tests (replace net core 3.1)