Fix import guards in NIOSSHServer and audit sendability

The `NIOSSHServer` example guards all of its code with `#if canImport(Foundation.Process)` to avoid build failures on platforms without `Process`.

However, Swift doesn't have submodules like this, Objective-C does. As Foundation transitions to Swift this check now always fails, even if Process is available. The result of this is that example code doesn't compile with newer Swift versions. Removing the can-imports results in a handful of Sendability warnings.

Start with the NIOSSHServer example and locate the code guarded by #if canImport(Foundation.Process). Build the example with a newer Swift version after removing the guards, then audit the reported Sendability warnings; done means the example compiles without those import-guard failures or unresolved warnings.

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