fix: Add `dpd_timeout_seconds` to `tunnel_preshared`

[aramatev]

Description

I think this is a simple typo, you guys are missing dpd_timeout_seconds in resource "vpn_connection" tunnel_preshared for both tunnels 1 & 2

Nothing to really test, these variables are missing and its clear they are in other types of vpn_connectino resources in your file except for tunnel_preshared.

As proof - https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/vpn_connection

Motivation and Context

Breaking Changes

How Has This Been Tested?

• [ ] I have updated at least one of the examples/* to demonstrate and validate my change(s)
• [ ] I have tested and validated these changes using one or more of the provided examples/* projects
• [ ] I have executed pre-commit run -a on my pull request

[aramatev]

@bryantbiggs please advise , can you accept this? it is missing from the module, and it keeps people from setting the timeout seconds in tunnels.

[aramatev]

@StephenKing, @nakopoulos-profile, @bryantbiggs bump, please advise, can we merge this in? do you really need me to add to examples? you've 4 types of vpn_connection resources and one of them is missing dpd_timeout_seconds for both tunnels

[aramatev]

@antonbabenko please advise if this PR is good?

[antonbabenko]

This PR is included in version 3.7.1 :tada:

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