fix: ipv6 egress route for single nat

[jbatmalle]

Description

After merged PR #1059, in the case of a single NAT with multiple private networks, only the route for the first network is retained; the others are removed. This behavior has changed.

Exemple

private_subnets = [
   "10.0.0.0/24", "10.0.1.0/24", "10.0.2.0/24",
   "10.0.3.0/24", "10.0.4.0/24", "10.0.5.0/24",
]
 
private_subnet_ipv6_prefixes = [
   1, 2, 3,
   4, 5, 6
]
enable_nat_gateway     = true
single_nat_gateway     = true
one_nat_gateway_per_az = false
create_egress_only_igw = true

Motivation and Context

I'm upgrading from v2.78 to v5.21. The issue was introduced in v5.7.1.

Why is this change required? What problem does it solve?

Otherwise, the first private route resource is retained (NAT gateway count equals 1), while the subsequent private routes are removed.

Breaking Changes

no

How Has This Been Tested?

I have tested on my code

Other thing

Could you create a new v5 version ?

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