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 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)

[coveralls]

coverage: 99.152% (-0.02%) from 99.168% when pulling f43c1c94e5e78ded68dd4a14c5cd0fd9df685274 on users/alexpeck/net9 into 56d0d7868c64451fc824eb45736ccafcca0dd452 on main.