Add default OS repositories for Ubuntu 26.04 (#4680)

Canonical currently plans to release Ubuntu 26.04 "Resolute" on April
23rd.

$ podman run --rm -it docker.io/library/ubuntu:resolute cat /etc/apt/sources.list.d/ubuntu.sources | rg -v "^#"

Refs https://discourse.ubuntu.com/t/ubuntu-26-04-lts-the-roadmap/72740

(cherry picked from commit 8e5a1b6)

Author: maximiliankolb

guides/common/modules/ref_default-operating-system-repositories.adoc

@@ -255,6 +255,22 @@ endif::[]
 
 GPG key: https://dl.rockylinux.org/pub/rocky/RPM-GPG-KEY-rockyofficial
 
+[id="ubuntu-26-04"]
+.Ubuntu 26.04
+[width="100%",cols="20%,34%,22%,12%,12%",options="header"]
+|===
+| Name | Upstream URL | Releases/Distributions | Components | Architectures
+| `Ubuntu 26.04 main` | `https://archive.ubuntu.com/ubuntu/` | `resolute` | `main` | `amd64`
+| `Ubuntu 26.04 updates` | `https://archive.ubuntu.com/ubuntu/` | `resolute-updates` | `main` | `amd64`
+| `Ubuntu 26.04 security` | `https://security.ubuntu.com/ubuntu/` | `resolute-security` | `main` | `amd64`
+ifdef::katello[]
+| `Ubuntu 26.04 Client` | `https://oss.atix.de/Ubuntu26LTS/` | `stable` | `main` | `amd64`
+endif::[]
+ifdef::orcharhino[]
+| `Ubuntu 26.04 Client` | see {atix-kb-clients} in the _ATIX Service Portal_ | `stable` | `main` | `amd64`
+endif::[]
+|===
+
 [id="ubuntu-24-04"]
 .Ubuntu 24.04
 [width="100%",cols="20%,34%,22%,12%,12%",options="header"]