Add versionless value for admin api and terraform (#5741)

This pr adds clarification on which value to input for dbt_version for
the admin api and terraform. Also raised in community slack. More
context in [internal slack
thread](https://dbt-labs.slack.com/archives/C05FWBP9X1U/p1720107316597949?thread_ts=1715782793.330549&channel=C05FWBP9X1U&message_ts=1720107316.597949)

website/docs/docs/dbt-versions/core-upgrade/01-upgrading-to-v1.8.md

@@ -21,6 +21,8 @@ dbt Cloud is going "versionless." This means you'll automatically get early acce
 
 Select ["Keep on latest version"](/docs/dbt-versions/upgrade-dbt-version-in-cloud#keep-on-latest-version) in your development, staging, and production [environments](/docs/deploy/deploy-environments) to access to everything in dbt Core v1.8 and more.
 
+To upgrade an environment in the [dbt Cloud Admin API](/docs/dbt-cloud-apis/admin-cloud-api) or [Terraform](https://registry.terraform.io/providers/dbt-labs/dbtcloud/latest), set `dbt_version` to the string `versionless`.
+
 ## New and changed features and functionality
 
 Features and functionality new in dbt v1.8.

website/docs/docs/dbt-versions/upgrade-dbt-version-in-cloud.md

@@ -17,6 +17,8 @@ By choosing to **Keep on latest version**, you opt for a versionless experience
 
 You can upgrade to **Keep on latest version** and the versionless experience no matter which version of dbt you currently have selected. As a best practice, dbt Labs recommends that you test the upgrade in development first; use the [Override dbt version](#override-dbt-version) setting to test _your_ project on the latest dbt version before upgrading your deployment environments and the default development environment for all your colleagues.
 
+To upgrade an environment in the [dbt Cloud Admin API](/docs/dbt-cloud-apis/admin-cloud-api) or [Terraform](https://registry.terraform.io/providers/dbt-labs/dbtcloud/latest), set `dbt_version` to the string `versionless`.
+
 ### Override dbt version
 
 Configure your project to use a different dbt Core version than what's configured in your [development environment](/docs/dbt-cloud-environments#types-of-environments). This _override_ only affects your user account, no one else's. Use this to safely test new dbt features before upgrading the dbt version for your projects.