Add 7.2.6 release notes (#474)

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

## Release Notes

* **Documentation**
* Published release notes for Unraid OS 7.2.6 with step-by-step upgrade
instructions, known issues documentation, and rollback guidance.
* Linux kernel updated to 6.12.87-Unraid to address critical security
vulnerabilities.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: “ljm42” <“larry_meaney@iname.com”>

Author: ljm42

docs/unraid-os/release-notes/7.2.6.md

@@ -0,0 +1,25 @@
+# Version 7.2.6 2026-05-08
+
+This release contains a Linux kernel upgrade to address CVE-2026-43284 and CVE-2026-43500, the Dirty Frag local privilege escalation vulnerability.
+
+## Upgrading
+
+For step-by-step instructions, see [Updating Unraid](/unraid-os/updating-unraid/). Questions about your [license](/unraid-os/troubleshooting/licensing-faq/)?
+
+### Known issues
+
+For other known issues, see the [7.2.5 release notes](/unraid-os/release-notes/7.2.5/).
+
+### Rolling back
+
+If rolling back earlier than 7.2.5, also see the [7.2.5 release notes](/unraid-os/release-notes/7.2.5/).
+
+## Changes vs. 7.2.5
+
+### Security
+
+- Upgrade the Linux kernel to address CVE-2026-43284 and CVE-2026-43500, the Dirty Frag local privilege escalation vulnerability.
+
+### Linux kernel
+
+- version 6.12.87-Unraid