docs: Update Client Agent IPv6 requirement (#1444)

Per [ICU-18250](https://hashicorp.atlassian.net/browse/ICU-18250), the
Client Agent's requirement to have IPv6 enabled was removed in version
0.20.0. This PR updates the following topics (click the link for a
preview):

- Updated the Important changes section of the version 0.20.0 release
notes: [Important
changes](https://unified-docs-frontend-preview-a8pxlrqom-hashicorp.vercel.app/boundary/docs/release-notes/v0_20_0#important-changes)
- Updated the main Client Agent overview topic to remove the note that
said IPv6 was required: [Client
Agent](https://unified-docs-frontend-preview-a8pxlrqom-hashicorp.vercel.app/boundary/docs/client-agent)


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

Author: Dan-Heath

content/boundary/v0.20.x/content/docs/client-agent/index.mdx

@@ -15,12 +15,6 @@ When the Boundary Client Agent runs alongside an authenticated Boundary client,
 If you enter a hostname that matches a Boundary alias that the client is authorized to establish a session to, Boundary automatically generates the session and transparently proxies the connection on your behalf.
 If the Boundary Client Agent cannot find an alias, or if there are any issues with authentication, network connectivity, or latency, the Client Agent defers DNS resolution to the previously configured DNS resolvers.
 
-<Note>
-
-You must enable both IPv4 and IPv6 protocols for your environment to ensure the Client Agent can start and perform DNS lookups.
-
-</Note>
-
 Currently HashiCorp tests the Client Agent on the following operating systems:
 
 - macOS 14

content/boundary/v0.20.x/content/docs/release-notes/v0_20_0.mdx

@@ -63,6 +63,18 @@ description: >-
     </td>
   </tr>
 
+  <tr>
+    <td style={{verticalAlign: 'middle'}}>
+    Client Agent no longer requires IPv6
+    </td>
+    <td style={{verticalAlign: 'middle'}}>
+    Previously, the Client Agent required you to enable both IPv4 and IPv6 protocols. Beginning with version 0.20.0, the Client Agent will no longer require IPv6.
+    <br /><br />
+    Learn more:&nbsp; <a href="/boundary/docs/client-agent">Boundary Client Agent</a>
+    </td>
+  </tr>
+
+
   </tbody>
 </table>