update custom headers to mention 1.9 is required (#13155)

* update custom headers to mention 1.9 is required

Per https://github.com/hashicorp/vault/blob/main/CHANGELOG.md#190-rc1 the custom response headers are a new feature introduced in 1.9, meaning we should explicitly call out this version requirement in documentation, otherwise users of earlier versions of Vault will unable to use the functionality and may consider it a bug.

* Update website/content/docs/configuration/listener/tcp.mdx

reads better, agreed

Co-authored-by: Loann Le <[email protected]>

Co-authored-by: Loann Le <[email protected]>

website/content/docs/configuration/listener/tcp.mdx

@@ -22,7 +22,7 @@ specify [`api_addr`][api-addr] and [`cluster_addr`][cluster-addr] so Vault will
 advertise the correct address to other nodes.
 
 ## Listener's custom response headers
-Vault supports defining custom HTTP response headers for the root path (`/`) and also on API endpoints (`/v1/*`).
+As of version 1.9, Vault supports defining custom HTTP response headers for the root path (`/`) and also on API endpoints (`/v1/*`).
 The headers are defined based on the returned status code. For example, a user can define a list of
 custom response headers for the `200` status code, and another list of custom response headers for
 the `307` status code. There is a `"/sys/config/ui"` [API endpoint](/api/system/config-ui) which allows users
@@ -260,8 +260,8 @@ listener "tcp" {
 
 ### Configuring custom http response headers
 
-This example shows configuring custom http response headers. Operators can configure
-`"custom_response_headers"` sub-stanza in the listener stanza to set custom http
+Note: Requires Vault version 1.9 or newer. This example shows configuring custom http response headers. 
+Operators can configure `"custom_response_headers"` sub-stanza in the listener stanza to set custom http
 headers appropriate to their applications. Examples of such headers are `"Strict-Transport-Security"`
 and `"Content-Security-Policy"` which are known HTTP headers, and could be configured to harden
 the security of an application communicating with the Vault endpoints. Note that vulnerability