Add v6 migration guide

website/docs/guides/upgrading_to_version_6.html.markdown

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+---
+layout: "selectel"
+page_title: "Upgrading Terraform Selectel Provider to version 6.0.0"
+sidebar_current: "docs-selectel-guide-iam-migrating-guide"
+description: |-
+  How to upgrade Terraform Selectel Provider to version 6.0.0.
+---
+
+# Upgrading Terraform Selectel Provider to version 6.0.0
+
+In version 6.0.0, Terraform Selectel Provider introduces deprecated API resources removal
+and some setup changes.
+
+Removed resources:
+
+- selectel_vpc_role_v2
+- selectel_vpc_token_v2
+- selectel_vpc_user_v2
+- selectel_vpc_vrrp_subnet_v2
+- selectel_vpc_crossregion_subnet_v2
+
+Before upgrading to version 6.0.0, [upgrade to the most recent 5.X version of the provider](https://registry.terraform.io/providers/selectel/selectel/latest/docs/guides/upgrading_to_version_5) and ensure that your environment successfully runs `terraform plan`. You should not see changes you do not expect or deprecation notices.
+
+## Required changes
+
+1. In the Terraform configuration(`.tf` files), update the version constraints:
+
+   ```hcl
+   terraform {
+   required_providers {
+     selectel  = {
+       source  = "selectel/selectel"
+       version = "~> 6.0"
+     }
+     openstack = {
+       source  = "terraform-provider-openstack/openstack"
+       version = "1.54.0"
+     }
+   }
+   }
+   ```
+
+2. Add required authentification parameters `auth_region` and `auth_url` to configuration if they don't exist:
+
+```hcl
+
+provider "selectel" {
+  domain_name = "123456"
+  username    = "user"
+  password    = "password"
+  auth_region = "pool"
+  auth_url = "https://cloud.api.selcloud.ru/identity/v3/"
+}
+
+```
+
+3. If you use environment variables `SEL_PROJECT_ID` or `SEL_REGION` you must rename them as `INFRA_PROJECT_ID` and `INFRA_REGION` respectively.
+4. To download the new version, initialize the Terraform configuration.
+
+   ```bash
+   terraform init -upgrade
+   ```
+
+## Optional changes (if you using deprecated resources)
+
+1. Backup the `.tfstate` file.
+2. Remove the `selectel_vpc_vrrp_subnet_v2` resource from the `.tfstate` file:
+
+   ```bash
+   terraform state rm $(terraform state list | grep selectel_vpc_vrrp_subnet_v2)
+   ```
+
+3. Remove the `selectel_vpc_crossregion_subnet_v2` resource from the `.tfstate` file:
+
+   ```bash
+   terraform state rm $(terraform state list | grep selectel_vpc_crossregion_subnet_v2)
+   ```
+
+4. In the configuration files, remove `selectel_vpc_vrrp_subnet_v2` and `selectel_vpc_crossregion_subnet_v2` resources.
+5. To ensure that Terraform has no unwanted changes:
+
+   ```bash
+   terraform plan
+   ```
+
+6. Apply the changes:
+
+   ```bash
+   terraform apply
+   ```