openvpn: T6591: deprecate OpenVPN server net30 topology (backport #3825)

[mergify]

Change Summary

Set the default topology to subnet for new configs, add a deprecation warning, and insert net30 in old configs explicitly for compatibility.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe): default change, deprecation.

Related Task(s)

Related PR(s)

Component(s) name

OpenVPN

Proposed changes

The net30 topology is:

• only there because it was the default in OpenVPN, but even then it was just for compatibility reasons, not because it's good;
• bad for Windows clients, they don't work with it;
• deprecated in OpenVPN and may eventually be removed.

We should set the new default to subnet and mark net30 as deprecated. But some people may rely on the net30 behavior, so we'll use a migration script to inject the old default into old configs explicitly.

How to test

Smoketest result

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

---

This is an automatic backport of pull request #3825 done by [Mergify](https://mergify.com).

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