Use System.Threading.Lock in ConcurrentLfu

[bitfaster]

Take the ConcurrentLfu lock optimization from #638.

Benchmark shows lock try enter is slightly faster:

Method	Mean	Error	StdDev	Ratio	Allocated
UseMonitor	17.88 ns	0.235 ns	0.220 ns	1.00	-
UseLock	14.79 ns	0.315 ns	0.309 ns	0.83	-

[coveralls]

coverage: 99.089% (-0.08%) from 99.169%
when pulling ae7ae9b on users/alexpeck/lock9
into fa14abd on main.

[Copilot]

Pull Request Overview

This PR optimizes the LFU cache’s maintenance lock by using the new System.Threading.Lock API when targeting .NET 9+, and extends project targets to .NET 9.0.

• Use Lock with TryEnter/Exit under NET9_0_OR_GREATER instead of Monitor.TryEnter
• Update main project to target net9.0 and bump C# language version to 13.0
• Extend benchmarks to run on net9.0

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
BitFaster.Caching/Lfu/ConcurrentLfuCore.cs	Added conditional compilation to switch from Monitor locks to the new Lock API
BitFaster.Caching/BitFaster.Caching.csproj	Added net9.0 target and updated <LangVersion> to 13.0, bumped validation baseline
BitFaster.Caching.Benchmarks/...Benchmarks.csproj	Included net9.0 in benchmarks’ <TargetFrameworks>

Comments suppressed due to low confidence (1)

BitFaster.Caching/BitFaster.Caching.csproj:5

• [nitpick] Bumping the language version to 13.0 might be ahead of official C# releases; consider using a supported preview or the latest stable version to ensure compatibility for all users.

    <LangVersion>13.0</LangVersion>