fix(netplan): handle ipv6_slaac separately from dhcp6

[armorbreak001]

The netplan renderer treats ipv6_slaac subnet type identically to dhcp6, generating dhcp6: true in the netplan config. This causes VMs to acquire DHCPv6 addresses in addition to SLAAC addresses, which is not the expected behavior for SLAAC.

Root cause: all IPV6_DYNAMIC_TYPES (which includes ipv6_slaac, dhcp6, ipv6_dhcpv6-stateless, and ipv6_dhcpv6-stateful) share a single code path that unconditionally sets dhcp6: True.

The other renderers (eni, sysconfig, NetworkManager) each handle ipv6_slaac as a distinct case:

• eni: sets iface inet6 auto with dhcp 0
• sysconfig: has a dedicated elif branch for ipv6_slaac
• NetworkManager: maps ipv6_slaac to method auto (distinct from DHCPv6)

This change makes the netplan renderer consistent: for ipv6_slaac subnets it now emits accept-ra: true instead of dhcp6: true. The netplan renderer already has plumbing for accept-ra at line 201 of netplan.py.

Fixes #6847

[github-actions]

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