Ignore changes to propagating_vgws of private routing table

[miguelaferreira]

This PR makes the routing tables for the private subnets ignore changes to their propagating_vgws.

When configuring VPN Gateways it is normal to define aws_vpn_gateway_route_propagation resources to automatically configure route propagation between the VPN Gateway and a routing table. However, without this PR, terraform will continuously propose changes to the infrastructure, adding route propagation in one run and removing it the next.

First run:

Resource actions are indicated with the following symbols:
  + create

Terraform will perform the following actions:

  + aws_vpn_gateway_route_propagation.private_subnets_vpn_routing[0]
      id:             <computed>
      route_table_id: "rtb-aaa"
      vpn_gateway_id: "vgw-xxx"

  + aws_vpn_gateway_route_propagation.private_subnets_vpn_routing[1]
      id:             <computed>
      route_table_id: "rtb-bbb"
      vpn_gateway_id: "vgw-xxx"

Second run:

Resource actions are indicated with the following symbols:
  ~ update in-place

Terraform will perform the following actions:

  ~ module.vpc.aws_route_table.private[0]
      propagating_vgws.#:          "1" => "0"
      propagating_vgws.3028428753: "vgw-xxx" => ""

  ~ module.vpc.aws_route_table.private[1]
      propagating_vgws.#:          "1" => "0"
      propagating_vgws.3028428753: "vgw-xxx" => ""

[antonbabenko]

Thanks for this PR, v1.13.0 is released

[github-actions]

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