Add deprecation notice to the 'legacy' network mode

[gbraad]

Over time, when both macOS and Windows have adopted the default use of vsock, we will rename the current network mode as 'legacy' or something similar ('vswitch', etc. This might therefore not see the same amount of testing as before and considered to become unsupported.

It would be best to indicate this expectation to the user. Even though this will still be an option, we will not be able to test this as much on these platforms.

[stale]

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