Should not be able to change IP subnets for a VPC Subnet

[bnaecker]

Looking at the code below

omicron/nexus/src/external_api/params.rs

Lines 251 to 260 in d4c11d2

	pub struct VpcSubnetUpdate {
	#[serde(flatten)]
	pub identity: IdentityMetadataUpdateParams,
	// TODO-correctness: These need to be removed. Changing these is effectively
	// creating a new resource, so we should require explicit
	// deletion/recreation by the client.
	pub ipv4_block: Option<Ipv4Net>,
	pub ipv6_block: Option<Ipv6Net>,
	}

we can see it's currently possible to update the IPv4 or IPv6 subnet for a VPC Subnet. That needs to be disallowed, where the only way to really do this would be to delete the VPC Subnet and create a new one with the desired parameters.

[bnaecker]

Closed by #1019