Remove synthesized offline state

[dlon]

The offline state should no longer be needed when #8125 is merged, as the local resolver is always running.

• [x] TODO: Mention in changelog that switching between networks will be more seamless.
• [x] Update default interface in split tunneling module
• [ ] Fix: DNS sometimes breaks when switching between networks.

Fix DES-2116.

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This change is 

[linear[bot]]

DES-2116 Remove synthesized offline state when switching between networks