NethVoice Proxy: Upgrade the SIP proxy to Kamailio 6.0 #8206

The NethVoice proxy currently uses Kamailio 5.8. Upgrade it to Kamailio 6.0 so new and existing installations can run the newer version.

The upgrade must update the database schema required by Kamailio 6.0 without losing existing data. It should be safe to run the migration more than once and preserve the documented ability to downgrade to Kamailio 5.8.

Acceptance criteria:

• A fresh installation starts Kamailio 6.0.
• An existing installation upgrades successfully and retains its data.
• Running the schema migration again succeeds.
• The upgrade and downgrade expectations are documented.