Remove "Optional" label from a prereq

Co-authored-by: Maximilian Kolb <[email protected]>

guides/common/modules/con_ipv4-and-ipv6-requirements.adoc

@@ -31,7 +31,7 @@ This is required because Red Hat Content Delivery Network distributes content on
 You must configure {Project} to use this dual-stack (supporting both IPv4 and IPv6) HTTP proxy as the default HTTP proxy.
 endif::[]
 ifndef::satellite[]
-* Optional: If you rely on content from IPv4-only networks and are deployed on IPv6-only, you can use an external dual stack HTTP proxy server.
+* If you rely on content from IPv4-only networks and are deployed on IPv6-only, you must use an external dual stack HTTP proxy.
 In this setup, you must configure {Project} to use this dual-stack (supporting both IPv4 and IPv6) HTTP proxy as the default HTTP proxy.
 endif::[]
 +