MemoryCache: use net9.cs file rather than #if for TFM limits

[mgravell]

Follow-up to #112695, addressing #if usage in /ref/ files

/cc @stephentoub

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

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• src/libraries/Microsoft.Extensions.Caching.Memory/ref/Microsoft.Extensions.Caching.Memory.csproj: Language not supported

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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[stephentoub]

Thanks

[stephentoub]

/ba-g unrelated failures