(Maybe) Multiple Network Rendering Bugs

These are my subjective judgments. They might bugs, or not. Not tested.
Developers can close this thread at any time.

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### 1

https://github.com/canonical/cloud-init/blob/d7eb828f8e232a1ce6d2fa66f8028a3b10adab1f/cloudinit/net/network_state.py#L406

If this line also applies to network config v1. The v1 types `ipv6_dhcpv6-stateful`, `ipv6_dhcpv6-stateless`, and `ipv6_slaac` results `false` here

Obviously, it is handled well here:
https://github.com/canonical/cloud-init/blob/d7eb828f8e232a1ce6d2fa66f8028a3b10adab1f/cloudinit/net/__init__.py#L1301

### 2

As far as I know, in the Subnet type of network configuration, `dhcp6` is equivalent to `ipv6_dhcpv6-stateful`.
However, in the following places, `dhcp6` and `ipv6_dhcpv6-stateful` are treated differently, resulting in different outcomes:

https://github.com/canonical/cloud-init/blob/d7eb828f8e232a1ce6d2fa66f8028a3b10adab1f/cloudinit/net/network_manager.py#L137-L146
https://github.com/canonical/cloud-init/blob/d7eb828f8e232a1ce6d2fa66f8028a3b10adab1f/cloudinit/net/network_manager.py#L167-L171
https://github.com/canonical/cloud-init/blob/d7eb828f8e232a1ce6d2fa66f8028a3b10adab1f/cloudinit/net/networkd.py#L211
https://github.com/canonical/cloud-init/blob/d7eb828f8e232a1ce6d2fa66f8028a3b10adab1f/cloudinit/net/sysconfig.py#L439

The last one I previously reported https://github.com/canonical/cloud-init/issues/4615
There might be similar issues elsewhere.

### 3
https://github.com/canonical/cloud-init/blob/d7eb828f8e232a1ce6d2fa66f8028a3b10adab1f/cloudinit/net/eni.py#L577-L581
Similar issues. `dhcp` is equivalent to `dhcp4`.
Should add `dhcp` here

### 4

https://github.com/canonical/cloud-init/blob/d7eb828f8e232a1ce6d2fa66f8028a3b10adab1f/cloudinit/net/sysconfig.py#L1119-L1121

`IPV6_AUTOCONF=no` is set here. It should be enabled by default.