fix(release-notes): correct 7.3.0 kernel version

- Purpose: make the 7.3.0 changelog show the current rc.2 kernel version.\n- Before: the Linux kernel section listed 6.18.23-Unraid.\n- Why that was a problem: the release notes conflicted with the intended current kernel version and the ZFS package reference.\n- What changed: update the Linux kernel bullet to 6.18.26-Unraid.\n- How it works: change the single stale version string in docs/unraid-os/release-notes/7.3.0.md.

Author: elibosley

docs/unraid-os/release-notes/7.3.0.md

@@ -190,7 +190,7 @@ To proactively switch to TPM-based licensing, see [How can I move from flash-bas
 
 ### Linux kernel
 
-- Linux kernel: update from `6.12.85-Unraid` to `6.18.23-Unraid`.
+- Linux kernel: update from `6.12.85-Unraid` to `6.18.26-Unraid`.
 
 ### Base distro updates