[runtime/6.0] [mono] Raise soft RLIMIT_NOFILE on Linux

[uweigand]

Manual backport of #82429.

Replace darwin_change_default_file_handles by a new routine increase_descriptor_limit, which mirrors the logic in the CoreCLR INIT_IncreaseDescriptorLimit routine.

Fixes #82428.

Customer Impact

Running dotnet restore using the Mono runtime (e.g. on the s390x architecture) would under certain circumstances abort with a "Too many open files" error message. The details are documented in NuGet/Home#12410.

It turned out that Mono does not raise the limit for open files like CoreCLR does. This change fixes that to align with CoreCLR.

Testing

Manual testing confirmed this fixes the problem.

Risk

Low, we're aligning the Mono behavior with CoreCLR and the affected codepath is only used on Desktop so doesn't impact the mobile/wasm workloads.

[carlossanlop]

@akoeplinger do you mind helping send the email to Tactics requesting approval?

[akoeplinger]

See the 7.0 PR.

[carlossanlop]

I'm retargeting this PR to the new release/6.0-staging branch, which is the one that we will use from now on for servicing fixes.

Repo maintainers will now be allowed to merge their own servicing PR as long as it meets the requirements:

• It is approved by Tactics (signaled by adding the Servicing-approved label).
• It's signed-off by an area owner.
• The CI is green, or the failures are investigated as unrelated.
• And if the PR touches an OOB package, the necessary OOB authoring changes are added.

The new process is described here: runtime/docs/project/library-servicing.md.

The infra team will be actively monitoring servicing PRs to ensure all requirements are met and to help with any issues.

Let me know if you have any questions.

[steveisok]

@uweigand as a general policy, we are only able to take servicing fixes for configurations we actively support. Since s390 is not one of them, unfortunately, we cannot take this change.