[VPN] Classic VPN - EoL-warning for related tf-providers - ONLY for Region eu-de

[uli-schneider]

Description of the enhancement

Regarding to OTC release notes:
Classic VPN End of Live in EU-DE as of 01.06.2025

The successor "Enterprise VPN" for Region eu-de is already available in tf-provider (_v5).
For Region eu-nl "Classic VPN" still continue and exists.

It would be helpful to have a "depricated" - "Warning" during terraform plan, if this VPN type is used in Region eu-de.
(similar e.g. VPC-Shared SNAT - Message)

Releated providers might be:
https://registry.terraform.io/providers/opentelekomcloud/opentelekomcloud/latest/docs/resources/vpnaas_endpoint_group_v2
https://registry.terraform.io/providers/opentelekomcloud/opentelekomcloud/latest/docs/resources/vpnaas_ike_policy_v2
https://registry.terraform.io/providers/opentelekomcloud/opentelekomcloud/latest/docs/resources/vpnaas_ipsec_policy_v2
https://registry.terraform.io/providers/opentelekomcloud/opentelekomcloud/latest/docs/resources/vpnaas_service_v2
https://registry.terraform.io/providers/opentelekomcloud/opentelekomcloud/latest/docs/resources/vpnaas_site_connection_v2

Please add also an hint in related tf-documentation pages, about EoL for this VPN-Type is Region eu-de is set.

If you have any question, please contact the related Productmanager or me.