Netplan renderer incorrectly mixes DHCPv6 and SLAAC

# Bug report

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 `ipv6_slaac`.

The root cause is in `cloudinit/net/netplan.py`. All `IPV6_DYNAMIC_TYPES` (which includes `ipv6_slaac`, `dhcp6`, `ipv6_dhcpv6-stateless`, and `ipv6_dhcpv6-stateful`) are handled by a single code path that sets `dhcp6: True`:

```python
elif sn_type in IPV6_DYNAMIC_TYPES:
entry.update({"dhcp6": True})
```

The other renderers handle `ipv6_slaac` distinctly:
- **eni** (`cloudinit/net/eni.py:538`): has a dedicated `elif subnet["type"] == "ipv6_slaac"` branch that sets `accept_ra` without enabling DHCPv6
- **sysconfig** (`cloudinit/net/sysconfig.py:467`): has a dedicated `elif subnet_type == "ipv6_slaac"` branch
- **NetworkManager** (`cloudinit/net/network_manager.py:141`): maps `ipv6_slaac` to `auto`, which is distinct from DHCPv6

The netplan renderer should generate `accept-ra: true` (which it already has plumbing for at line 201) and `dhcp6: false` when the subnet type is `ipv6_slaac`, rather than setting `dhcp6: true`.

## Steps to reproduce the problem

1. On Proxmox VE, create a VM with cloud-init and set IPv6 to SLAAC:

```
qm set --ipconfig0 ip=10.0.0.100/16,gw=10.0.254.253,ip6=auto
```

2. Regenerate the cloud-init image and inspect it. Proxmox generates the correct v1 network config on the cloud-init ISO:

```yaml
version: 1
config:
- type: physical
name: eth0
mac_address: 'bc:24:11:34:de:85'
subnets:
- type: static
address: '10.0.0.100'
netmask: '255.255.0.0'
gateway: '10.0.254.253'
- type: ipv6_slaac
- type: nameserver
address:
- '10.0.0.1'
```

Note `type: ipv6_slaac` -- this is correct.

3. Boot the VM (Ubuntu 24.04 with netplan renderer) and inspect the generated netplan config:

```bash
cat /etc/netplan/*.yaml
```

**Expected:** `accept-ra: true` and either `dhcp6: false` or no `dhcp6` key.

**Actual:** `dhcp6: true`, which causes the VM to solicit DHCPv6 addresses in addition to SLAAC addresses.

```
$ cat /etc/netplan/50-cloud-init.yaml
network:
version: 2
ethernets:
eth0:
match:
macaddress: "bc:24:11:34:de:85"
addresses:
- "10.0.0.100/16"
nameservers:
addresses:
- 10.0.0.1
search: []
dhcp6: true # <-- should be accept-ra: true, dhcp6: false
set-name: "eth0"
routes:
- to: "default"
via: "10.0.254.253"
```

On a network with both SLAAC RAs and a DHCPv6 server, this results in the VM acquiring unexpected DHCPv6 addresses. In our case, SSSD's `dyndns_update` then registers these extra addresses in DNS, causing intermittent connectivity issues as clients resolve to the wrong IPv6 address.

## Environment details
- Cloud-init version: 25.3-0ubuntu1~24.04.1
- Operating System Distribution: Ubuntu 24.04 (Noble)
- Cloud provider, platform or installer type: Proxmox VE 8.x (NoCloud datasource)

## cloud-init logs

The relevant output is the generated netplan config. No crash or error occurs; the issue is silent incorrect behavior. The cloud-init input (from the NoCloud ISO mounted at `/dev/sr0`) and the netplan output are shown in the reproduction steps above.