provide a net9 build target

[bitfaster]

• Use new System.Threading.Lock type in ConcurrentLfuCore. Benchmark shows Lock.TryEnter is faster.
• POC of TryGetAlternateCache for string keys, see Add string keyed dictionary ReadOnlySpan<char> lookup support dotnet/runtime#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)

[coveralls]

coverage: 99.152% (-0.02%) from 99.168%
when pulling f43c1c9 on users/alexpeck/net9
into 56d0d78 on main.