docs: Remove remaining IPv6 requirement (#1465)

For [ICU-18250](https://hashicorp.atlassian.net/browse/ICU-18250), we
removed documentation that said IPv6 was required for the Client Agent,
as this requirement was relaxed in version 0.20.0. We merged the changes
in #1444 , but I missed a requirement listed in the [Configuring
transparent
sessions](https://developer.hashicorp.com/boundary/docs/targets/configuration/configure-transparent-sessions#requirements:~:text=Ensure%20that%20both%20IPv4%20and%20IPv6%20protocols%20are%20enabled%20for%20your%20environment.%20The%20Client%20Agent%20requires%20both%20protocols%20to%20start%20and%20perform%20DNS%20lookups.)
topic. This PR removes that reference.

[View the preview
deployment](https://unified-docs-frontend-preview-abhsle72z-hashicorp.vercel.app/boundary/docs/targets/configuration/configure-transparent-sessions)

[ICU-18250]:
https://hashicorp.atlassian.net/browse/ICU-18250?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: Dan-Heath

content/boundary/v0.20.x/content/docs/targets/configuration/configure-transparent-sessions.mdx

@@ -28,7 +28,6 @@ Before you configure transparent sessions, you must:
 - Install the Boundary Client Agent, CLI, and Desktop client.
    - Refer to [Install the Boundary clients](/boundary/docs/deploy/self-managed/install-clients) for instructions.
    - Refer to the [transparent sessions interoperability matrix](/boundary/docs/interoperability-matrix) for a list of third-party products that have been tested for use with the Boundary installer and Client Agent.
-- Ensure that both IPv4 and IPv6 protocols are enabled for your environment. The [Client Agent](/boundary/docs/client-agent) requires both protocols to start and perform DNS lookups.
 
 You should also make sure you have the appropriate grants configured so that you can resolve aliases and establish transparent sessions.
 For more information, refer to the [Grants](/boundary/docs/client-agent#grants) section in the Client Agent documentation.